tell me to stay out of it.

I understand... Hard to sit back and watch.

If he’s an adult, I probably would tell him. A kid? I’d just bite my tongue.

Need to know how old he is   And the kind of relationship you all have I guess.

How could your sister have made sure your sons were included? I presume they're well into adulthood too? I get that it's hurtful and spoiling familial events etc but he is entitled to make different decisions surely. Is his wife in contact with any of you individually? Are the changes linked to her at all?

What is he doing that makes you say he's being a shit to his parents?

I’d stay out of it. You’re clearly angry and that’s not going to help anything.

Usually, people become estranged from their family for complicated reasons and it’s rarely 100% on them. Telling someone with an already strained family relationship that they’re wrong about everything isn’t going to heal anything.

I definitely need more specifics on what he's doing currently.

I took it as he participated in the past and, this year, he said he wouldn't be participating. If that's correct, I don't see that as rude. Why should he have to participate if he doesn't want to? If anything, I think it's rude to expect someone to exchange a gift, when they don't want to. 

So far, I haven't read anything that paints him as bad person.

Rehearsal dinners normally include the wedding party and out-of-town guests. They are entitled to make their own choices about whom to invite (along with whoever is actually footing the bill, if it isn’t them), but it is both rude and unkind to invite the other cousins and leave out your two sons.

I don’t see how skipping the gift exchange if they don’t enjoy it is terrible or anything, but it does hurt other family members’ feelings, and he should realize that and do it gently, not (again) rudely.

This sounds like the tip of the iceberg on family issues with this guy and maybe his wife, too. I would try to stay out of it, but I doubt things will improve any time soon.

Feel free to vent here all you like, though.

100% this. We had a very small wedding but we did include several out of town family members to join us for the rehearsal dinner because it’s the polite thing to do.

Since you don't want to share any details of why he's a shit other than a rehearsal dinner where you cried because your sons weren't invited, and a Christmas exchange of cards he didn't want to do, I'll just cut to "stay out of it".

Sounds like a bunch of unnecessary drama.

ETA and his rehearsal dinner is not YOUR ground to stand, nor your sister's. It's not your wedding.

I agree. Everyone says set boundaries with your family, and also that the wedding couple gets to do what they want and not what’s expected…and then when they do. It seems weird to be upset about a gift exchange boundary and them doing what they wanted to for HIS wedding.

Hopefully not sidetracking this thread, but the idea of ‘because it’s the polite thing to do’ really bothers me. I think it’s a regional/cultural/maybe socioeconomic idea to invite ALL the out of town guests to the rehearsal. In my world it is common to have only the wedding party participants and their immediate family at the rehearsal dinner. This caused a huge issue with my MIL who only wanted to invite out of town guests from her side to our rehearsal when I wanted to keep it small with only wedding party and their families/spouses. It’s ok to not invite the entire world. Period. I’ve also traveled for several weddings and never been invited to a single rehearsal dinner. INCLUDING those hosted by said MIL (eye roll).

It sounds to me like the nephew is trying to set some boundaries. With lack of additional information, I say good for any person who attempts to set boundaries in their lives.

Definitely stay out of it. You can be a listener for your sister. Nothing you say to someone like this will help.

I don't think the reasons you gave (or those types of reasons) are enough to confront your nephew with general involvement.

--The gift card thing is just a preference/boundary. He absolutely has the right to that choice.
--The rehearsal dinner is more concerning, but there are so many variables I don't know. If EVERY other cousin was invited and those cousins also didn't have a role in the wedding other than being an "out of town guest" then I would be annoyed, and I would probably ask why (or have DSs ask why). If the families are "close" then I am thinking there is an issue between your nephew and his cousins?

Either way, I think you have the right to be a bit miffed about the wedding, but being annoyed with him isn't enough to upend your already tense relationship. --coming from a person who has experienced this in a roundabout way and now have a strained relationship with MIL and SIL (it came from their end and DH has mended some with MIL but it is still tense and has little to do with his sister.) We get along with everyone else in the family, but MIL and SIL burnt some bridges with their "perceived" slights. Bottom line: MIL "thought" she was doing the right thing, but she really didn't understand the bigger picture and thought she had more sway with her opinions over her 50-plus-year-old sons than she did.

I am 95% sure your nephew would not appreciate your advice.