Is it right to throw yourself a baby shower for your 4th child?

My perfect sister-in-law and her husband are throwing themselves a baby shower for their 4th child, just because it is their 1st girl. Granted, there are 6 other female kids in the family, so it’s not like its the very first grand-daughter. This is also someone who throws her own birthday parties. I don’t want to attend, but I know if I don’t, it’s going to cause some stress with my husbands family. (This is the same family that kicked us out of their house when were trying to save up money to move. We were there for 6 months and they booted us to make room for this same sister and her kids. We were contributing to the house, paying rent, buying food and I was doing most of the cleaning!) His parents dote on his sister. She can do no wrong. She got pregnant to this man before they got married (while he was in his breaking her bones stage) and had an abortion. Now she wants us to celebrate this kid. I know that this is just her way of getting gifts. They are well off (new house, 2 new cars, vacations all the time) so its not like she needs presents.
I just might be feeling upset because no one in that family threw anyone else a baby shower (except the other sister) and the only time we are asked to attend things is when there is gifting involved. My gut feeling is “bugger his family”. I really can’t stand her “better than the rest of us” attitude and don’t’ really want to be around her or her friends. We have never forgiven him for beating on her either. The whole situation makes me want to puke.
Anyone want to tell me I am justified in being upset?