How To Handle Invitation For My Nieces` Sweet 16

My partner, her daughter (my step daughter) and I, were invited to my niece`s Sweet 16. I have almost never shared with my niece throughout the years because the family has held issues since I am tg, and we are an alternative family. This time, somehow we were "partially" invited.

My partner has two kids a girl15 and a boy 27. They know them both since 1992 and 1985. However, her elder son (my step-son), was not invited. I spoke to my niece and told her we were not going and why. She just said, "I`ll mail you the invitation, but he cannot come because, "I do not know him." Since that information was inaccurate we felt disappointed. We feel he probably would not have come because of his busy schedule at work, but we were offended by her wanting to leave him out. We would have mailed her a return lettter stating how many of us were attending.
My brother called and stated that he did not like what was happening and that he was going to talk to his wife. Next, I receive a call from his wife stating to disregard the invitation. Well, we were not going anyway. We think this was handled incorrectly.

We feel both our son and daughter should have been invited with perhaps a note requesting how many of us were coming. Would like to know proper ettiquette in a situation like this. My brother`s wife never had a good relationship with me ever since she fell in love with my brother more than 20 years ago. I feel they should have not extended an invitation if they were not able to include us as a family.
In addition, now I have my sister`s 15 year old son stating that I should sacrifice and go because it is her day. I believe they should have been good host and not try to separate the family and if she wanted me there, which I would have loved to go, then she needed to invite all four of us and then request who is coming.
Offended