That fact that you started with you wouldn’t have “chose” him for her, Makes me think you are possessive over her. Why would it cross your mind to chose who your sister is involved with in a romantic way. Most couples call each other baby like it becomes their name. Have you asked the rest of your family how they feel? If you are the only one who feels strongly about it perhaps you are jealous? I don’t know. Not trying to be rude here. Just trying to offer you a different perspective. The only part that I think mattered in what you said is the fact that she won’t play games if he doesn’t, or would rather help him win then help herself. Everyone needs to be a little independent. Other things to consider, how much has he been around your family before this? Maybe he has expressed being a little nervous to her and so she could be trying extra hard to make him feel like he’s not left out. Maybe he is shy? As far as the wedding....... I get to chose my best friends to to my wedding party. I would not feel obligated to have someone’s sister in my wedding. I’m here to unite with one person, the people standing beside ME are here for ME, just like they people standing for HIM are for HIM. And as far as their social life, that’s their life. It is healthy to establish your own activities in life. I believe when you find your wife you will understand more.

I don't think that asking your sibling who is at the height of the 'honeymoon period' of her romance to stop being so affectionate or stop using pet names for her fiancé is likely to significantly change her behavior or to have a positive or neutral affect on your relationship with her. It does sound like she is predisposed to view any attempt to ask her to tone it down as an attack on herself and her relationship, and I don't think there's much you can do interpersonally to completely avoid that.

I can't say with any confidence that this is definitely what's going on, but I would venture to speculate that your disdain for her behavior is visible to your sister, and possibly appears to her as disdain or disgust for her fiancé or their love/relationship. If this is true, it seems pretty obvious that the logical response from her perspective would be to limit your involvement in wedding-related matters, which could account for not being part of the wedding party. The other likely outcome is her doubling down in publically expressing that Dave and his feelings are more important to her than the opinions and feelings of all the 'haters'. You see this a lot in teenagers who are dating someone their friends or family disapproves-- an excessive need to defend their couple-hood and build up a sense of 'us against the world'. I've seen it, and, embarrassingly... done it, before.

It may help to accept that your sister is probably not trying to be irritating. It could be the defensive behavior that I mentioned above, it could be an uncontrollable outpouring of mushiness because of the upcoming wedding, or it could just be that she's developed some habits around how she communicates with Dave. There is a good chance that if it's infatuation or defensiveness, that will fade out gradually after the wedding. If it's just an irritating habit, who knows.

If you want to talk to her about it, the approach that is widely recommended in both business and personal settings, is to 'wrap' a criticism or a behavior change request in positives, and focus on the behavior that you want to change, not the person doing the behavior. One description of this skill is found at https://personalexcellence.co/blog/constructive-criticism/. You can also 'take the blame' a bit by saying that change is needed due to your own foibles, (rather than that her behavior is objectively obnoxious).

Hey sister, you know I think Dave is an awesome guy, right? I'm glad you two found each other. So, I've been struggling with how to bring this up. It's not your fault, but I am really uncomfortable with public displays of affection. Could you humor me and try to keep that in mind when we're hanging out? Thanks, sister. You've always been like a best friend to me as well as a sibling, and I love seeing you so happy with Dave.

Be specific about what aspects of her behavior bother you, and phrase it as a case of you being uncomfortable (rather than her being annoying). For example, "I feel like the game isn't as exciting when you and Dave are basically one team and the rest of us are single players" Is better than, "You ruined the game when you helped Dave." Adjust to suit your communication style and address the behaviors that bother you the most.

We are only hearing your perspective on this, which you have to admit may be skewed slightly due to your sibling relationship with your sister. That said, I can't argue with the factual situations you have described and I think most people will agree that her seeming rejection of her own family in favour of her future in-laws, along with an apparent desire to please her partner more than herself is a little weird and could indicate some form of obsessive behaviour.

I wouldn't say she has the typical signs of 'Relationship-Obsessive Compulsive Disorder', as this tends to manifest itself as negative obsessions and has a debilitating effect on people. However it does seem like a kind of obsession.

Doctor of Medicine Roxanne Dryden-Edwards writes:

"the person who suffers from obsessive love tends to want to spend excessive time with their love object, such that they think excessively about and engage in behaviours that put them in touch with their love object to an extreme degree. They may limit how much they engage in recreational activities or other social relationships, even becoming incapacitated to the point of being unable to work"

Your sister's behaviour fits this somewhat, with her family relationships being minimised and her not wanting to engage in any activity without her partner. However, the doctor goes on to write that Obsessive Love is so strong as to require psychotherapy treatment, and as you do not mention any concern about her becoming "incapacitated" I suspect that this is not quite this bad yet.

The same article I quote says of healthy love that:

"A healthy love relationship usually evolves over time such that it no longer involves the near desperate intensity and fervour of infatuation. Healthy love tends to mature over the years to include commitment, friendship, and a solid respect for the other person as an individual and of their needs."

If your sister's behaviour is nothing quite as serious as "obsessive" love then this would suggest that her behaviour will change over time as her relationship matures. True, she has been with him 2 years, but being engaged to be married may have reignited her infatuation with him at this time.

If you aren't prepared to just wait and allow your sister to enjoy her wedding and then settle into marriage, and you feel that you absolutely must say something to her about her behaviour then choose the focus of your statement wisely. If you make it about the public displays of affection then she is likely to compare what you think with what other people think, and as she seems to be surrounding herself with his family who might well think such displays are fine, you might just find yourself dividing the two families further. As all these behaviours are connected, could you perhaps encourage her to do something for herself the next time she passes up an opportunity because her partner will not be there? It could help her realise that she has lost herself a little in this relationship.

You might say:

Are you sure you don't want to do [x]? I can understand you wanting to be with Dave, but I'm sure he does things for himself without you sometimes. You need some time for yourself in a healthy relationship.