Are reflexive pronouns typically used in compound objects?

My short answer is that both are acceptable. The following attempts to justify this answer syntactically.

As an aside, let's note that you can say:

• I bought drinks for me! [e.g., not for anyone else on the trip]

...which makes it clear that the reflexive is never strictly syntactically necessary. However, turning to the spirit of your question, I would say that both your options I bought drinks for my sister and [me / myself]:

• sound fine to, and are used by, many native speakers, although obviously individuals may tend to use one or the other more
• are in fact both grammatically flawless

The first point is self-evident, I believe. I would motivate the second point by analysing the two sentences differently:

• [I [bought [drinks] [for [my sister and me]]]]
• [I [bought [drinks] [for [my sister] and [... [myself]]]]]

Don't worry about all the brackets; the important thing to note is just that [my sister and me] is a single constituent in the first sentence, and so does not have a reflexive form. In the second sentence, [myself] is a single constituent, and so can replace [me] (or, strictly speaking, can realise the first-person singular) in the standard manner of reflexives. Note the ellipsis---I assume the underlying structure is something like

• I bought drinks for my sister and I bought drinks for myself

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Having said all this, I would like to suggest another, completely different reason why the myself version is acceptable. This is that reflexive pronouns, at least in British English, are becoming an option to replace accusative pronouns (like me, you) in speech, possibly when mild emphasis is required:

• I just wanted to check this e-mail with yourself before I send it

To me this sounds horrible, but I hear it all the time, especially among London businesspeople. (I believe it is also traditional in some forms of Irish English, where it does not sound horrible to me!)

One can reject this particular use of the reflexive forms while recognising that it may be contributing to their greater prevalence in other constructions.