Dealing with flatmates who do not care for cleanliness

I've been on both sides of this, and in my experience, the person who wants things cleaner will need to do more cleaning or deal with things not being as clean as they'd like. This may not seem fair to have to clean other people's messes to meet your standards, but on the other hand, it's also not quite fair to force people to clean more than they would have to if nobody minded their mess. So try to keep some perspective that it's going to be a sacrifice for somebody no matter what.

I've found the most effective method to stay sane is to pick your battles. The closer you are to your housemates, the more you can ask of them, but you're unlikely to get everything you want even if they're your spouse. So consider if there are things that really bother you to clean or if they can do something easy to minimize the impact of the mess. For example, maybe you don't complain about them putting recyclables on the counter instead of the recycling bin, but you do ask them to clean up their hair in the bathroom every day. Or maybe you can have them quickly rinse the dishes after eating so they don't smell while they sit and are easier for you to clean or designate a small area for dirty dishes so they don't lay around everywhere.

If they feel like no matter what, they'll never please you, then they may decide not to try at all. If they take out the garbage and do the dishes promptly and wipe down the shower after using it and you still complain about the toilet, they'll just give up. I'd pick no more than three things to ask of them, and I'd make one or two of those things easy to do. Whenever you see they've done something you've asked, thank them so they know the effort is being appreciated. You may feel it's ridiculous to thank an adult for cleaning up after themselves, but remember, they could choose to not to do it. They're cleaning for you.

I said earlier that what you can expect will depend a lot on your relationship. A spouse will care more about your happiness than a random roommate who doesn't even like you. The latter may require you to offer them something in exchange for them to make any effort at all. It sounds like in your current situation that your partner's sister is the issue. You may want your partner to be the one to ask her to clean up more, since she's more likely to care about her sister than you, unless you're unusually close.

Sometimes you may share living quarters with someone who does care about keeping things clean enough to satisfy you, but they simply don't realize what is bothering you. I had a conversation with my boyfriend where I complained that he doesn't help clean and listed many things I had to do myself. His response was that he just doesn't notice those things and I need to ask him to do them. I felt like that was a cop-out, but sure enough, he will happily do chores when I ask him. I've also found I sometimes need to be more specific with a request. Instead of "clean the bathroom" I might say "clean the bathroom including the mirror and floors." Some people just don't see messes. Be careful you don't become too demanding or condescending though, watch their reaction and back off if they're getting grumpy.

The best way to prevent this situation in the future is to set expectations before you live with someone. Talk about things like cleanliness, noise, guests, and sharing items before it becomes a conflict. Everyone can put on the table what's important to them about their living space and agree on what they feel is reasonable. It prevents resentment and people feeling blindsided: there's a question on this very site of someone complaining they wouldn't have chosen to live where they do if they'd known rules about cleanliness and storing common items would be so strict. People are more agreeable about doing things that they, you know, actually agreed to. If you can make that conversation mutually beneficial, then you could try having it now, but I wouldn't if you think they'll feel coerced into agreeing with you..

In short, set clear expectations, pick your battles, and use what motivates the person. Good luck!