How can you not come to a 9 year old birthday party but come to his sister's!!!

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I'm so mad. My nephew is 9 today and his mom is having a bowling party ( she paid for everyone to come) well everyone told my sister they can't make it because they're having a family reunion but they knew today was his birthday and never invited them to the family reunion his grandpa is great grandpa his aunt is the one throwing it so my nephew has only us coming to his birthday most of us are adults there is maybe three kids coming one being his sister his cousin who he sees every day and one other cousin maybe. But for his sister's 4th birthday party at the same bowling alley his dad's side of the family came ( the ones doing the reunion) and more people from his mom side of the family came. They do this every year to him they make up excuses on why they can't come but yet will show up to his sisters he is special needs so it gets to him a whole lot more then it would get to most kids. Next year I will have my son who will be turning one ( going to be induced tonight so mine should be here tomorrow or Monday) and I will be doing a joint birthday party just so people will come to my nephew's birthday party. I just think it's so wrong that he has to suffer knowing nobody can come to his but they will come to is sisters and his cousins ( they're really close.) birthday parties. He has no friends really outside of family. But yet we have a huge and I mean huge family and only about 3 other show up every year. It's getting ridiculous. Most of the family has not come to any of his birthdays but all come to the other two kids birthdays.