Depends on who else was invited and what was the scale of the party.

Hmm, interesting, I'm thinking there's lots of ways to read a gesture like that - She doesn't like you and is rubbing it in by choosing your sister even though she knows she wouldn't know your sister at all if not for you marrying into the family. She's got a separate relationship with your sister that isn't connected to you. You could've ended up with her as a sister-in-law as she was friends with your sister first before you / she married into the family and became sisters-in-law. She's your sister's sister-in-law as well remember so she might consider it an entitlement. She might have thought it wasn't the kind of party you'd like to go to but your sister may want to. I guess as to whether or not you should feel offended, that depends on how you view your relationship with your sister-in-law. Your sister will always be your sister and unless she's an enemy of yours, vice-versa or both, you won't be losing your sister any time soon. Can you ask your sister about this and find out from her why you didn't get invited? Would you prefer your sister not go and would you feel comfortable and as though you had the right to tell her you didn't want her going? Do you think your sister should refuse the invite? Did she accept the invite anyway? Can you talk to your sister-in-law about this? Not all in-laws get along (I know, understatement of the year). Personally, I'd probably feel somewhat snubbed but then as said, there's lots of reasons why this happened. I hope you feel better soon as I'm guessing you are feeling offended or you'd not be asking about it as a question - good q though. It's an interesting situaiton. Regards, Meg