I don’t have enough information to comment on whether your husband is “over-attached.” He may simply be “attached,” which is a good thing. A man who is loving and loyal to his family is usually a man who knows how to be loving and loyal to his wife. If his family was asking him to choose them over you, it would be another story. I don’t see that in your letter. Instead, I see a family that misses him and that tries to pack 12 months of contact into 1 month.
You didn’t mention how old his sister is. If she is over 21 and inappropriately hugging, it might be advisable for your husband to start asserting some boundaries. That’s a “might.” Since I can’t witness what goes on, I can’t venture a definite. If she has a boyfriend or is dating, it’s probably not a problem but if her only affectionate contact with a male is your husband, it may not be helpful for him to go along with all of it.
If you were seeing me as a couple, I’d be asking your husband what he can do to reassure you that he hasn’t “forgotten” you during his family visits. And I’d be advising you to make more effort to befriend the sister by taking her on some “girls’ only” lunches or shopping trips or events. You’ll learn more about her when she isn’t so focused on her brother. It would be helpful if your husband supported you by encouraging his sister to go with you.
I wish you well.