@Mamana127 I have been and still am in the exact same place with one of my stepkids and his partner. However they don't send cards or gifts to either me or my husband (their dad). Every year I send joint (as in from me and their Dad) birthday and Christmas cards and joint gifts. We get a thank you text if we are lucky but they do not reciprocate even with a card.
I was nothing to do with their parents split, there was no extra-marital relationship on either side, it just didn't work. I came along years later, I get on well with 2 of his adult children and ironically his ex wife. The 3rd child and his partner are the problem.
When we see each other at family gatherings they seem fine, happy to see us, but out of sight, out of mind.
I told my husband last year that from now on I am mirroring their actions. If we get nothing, they get nothing. My husband was not impressed and we argued over it. He sent them cards and gifts but sent them from us both. I told him not to add my name but he did. This year, we got ignored as usual and so this year he just sent them cards. I think he maybe coming over to the dark side.
So ask your husband why mirroring what they do is wrong and that he needs to stop being a coward and address the elephant in the room with his kids if he wants YOU to have to keep dealing with rejection because you have continued to try and he has not supported you. They seem happy to take you for all you will give but show no ounce of appreciation, reciprocation and therefore respect for the family dynamics. They constantly do this and you have continued to try to be civil but there comes a time when enough is enough.
They are adults not kids, they have made it clear where they stand and the MIL has also told you. Tell your husband that despite your many years of efforts, they continue to slight you and so you are finally accepting what they want of you and are acquiescing. You have followed THEIR lead, if he has a problem, he needs to talk to them and not you.
However I would also say the fact that you do not send joint presents, reinforces that he does not support you because it allows them to ignore you specifically and I wonder if they would do it if it was joint presents. Your husband is not thinking about the scenario he is helping to make and he needs to rethink the situation.
Big hugs!