Feeling hurt, should I say something?

Why not find out? Looks like you have nothing to lose by asking how you have upset her, pleasantly though (you are by your own admission a bit tactless so be careful!) Maybe ask her out for lunch/coffee?

I wouldn't give the SIL the pleasure of knowing that you were upset.

Me neither.
‘Keep calm and carry on’.

Either continue to tolerate her snide remarks or learn to just let them wash over you. The choice I'd up to you. We don't all get on with everyone.

Just ignore her poor behaviour. Take the higher ground and be as polite as you can when you come into contact with her. It really isn't worth the drama and hassle of trying to communicate with someone like her.

If she has "issues" then you aren't going to be the person who can help her resolve them unfortunately.

Ignore her
She has the problem.. Not you

Remember this, what goes around comes around. Bide your time and you may have the last laugh. Don't let anyone in the family know this has hurt you, that would be playing into their hands.

You can't win a game like this! Who knows why she dislikes you - she probably doesn't even know herself. You will never make her like you. Just ride above it. Paint a smile on your face and go to the party!

Oh I totally agree with previous posters let it ride over you and enjoy you’re relationship with all the others
I can understand your hurt at being left out I would be hurt too it was nasty but it’s past now give your positive vibes to all those who do care

What vampire and urmston say is good advice. Let it lie. She probably won't have a coherent answer to such a question anyway. Read her behaviour.

Maybe your younger brother is right.

Shrug and carry on being you.

I agree. It's quite likely, from what you say, that she would wish you to feel the slight. So if you appear oblivious to it, you will have twice the satisfaction. You will know you behaved gracefully AND you will have won that particular little episode.

Are the other aunts your SIL's own sisters? If they are, perhaps your younger brother is right about her preferring to be surrounded by her own family... If, by your own admission, you can be tactless, perhaps you touched a particularly sensitive nerve at some point. It may be worth asking her in those sorts of terms...if you feel it is not already too late.....but, perhaps, NOT couched in terms of your hurt about the day's outing....

I tend to agree with Lukka. I would go up to the woman and ask her straight out what you have done to offend her. This will probably make her feel uncomfortable. She may tell you, or may not. Either way, you've got nothing to lose.

I would be very tempting to “call her out” next time. Something like “Do you find it easy to be so unpleasant, or do you have to work hard at it”?
Said, of course, with a beatific smile on your face ...

I disagree, she is a thick skinned sociopath and she won't feel a damn thing. However if you glide in and carry on, she will be raging. Oh and make up some fun thing you did while they were in their clique.

Some sound advise as usual on Gransnet.
Have you thought of inviting your niece out for afternoon tea just the two of you as a special bithday treat, much nicer than sharing her and being on eggshells.
Enjoy party

My stock answer to catty remarks is did you say that to be funny or just unkind?

I'd just ignore it completely. I wouldn't allow somebody like that to upset me in the first place.

I'd have a low opinion of them, obviously, (but keep it to myself) so anything they thought, did or said wouldn't matter to me. Why would I care whether they liked me or not?

If the situation is that bad then you don't lose anything by asking her if you've done something to offend her but if it's something you can put up with then do so with a smile on your face. I do wonder if your niece knows you weren't invited to the earlier get together.

You could ask the others,at the party,( the 1s who went on the outing?) Why you werent asked as well? See if their 'reasons' tally? As for the SIL 's comment about 'having a lot on'- is this where its stemmed from? Has something occurred which you have not realised,and offended her?- maybe another relative needed taking to an appt or somewhere for example & you didnt offer so theyve had to step up maybe? Hence the ' weve all got a lot on' comment?try think back & see? If not just ask her what(or if?) you have have done to offend her? Then answer her according to what it is.If you havent then say you would like to get along better with her& be included next time? Clearly theres something happened if she used to be ok with you previously?

When someone is continually unkind for no apparent reason it’s often because they’re jealous! I always told my children this when they were growing up and it helped them to feel better about themselves. Sadly it’s your sister in law’s problem - don’t make it yours. X

I also like marpau's reply.

What a horrible situation for you - I can sympathise fully.
However, through bitter experience I have learned that you rarely get anywhere with 'clearing the air' - having tried this there was a complete denial of all nastiness and I achieved nothing. (Except to give proof that I had been upset and noticed what was being said and done) I told myself then that the awful behaviour said a great deal more about the other person than me, and although I couldn't alter her attitude I could modify mine. I retained a cheerful, don't care attitude and at social gatherings I was light hearted and friendly to all. Gradually the poisonous behaviour didn't matter to me any more, and years later several family members commented on what a nasty piece of work this person had been - and seemingly it was not just to me. You sound like sensitive and kind woman, and often jealousy is at the root - ignore her, laugh, and enjoy your family times - the very best revenge.

Ignore. It would likely be denied that there is any issue anyway.