Vent and need advice

Quick backstory: my mother passed away over three years ago and when that happened, I moved in with my brother, his gf and their baby. Throughout that time his gf and I became kind of close and she would always say how she planned to put me in their wedding as a bridesmaid. 
Side note: I have been babysitting their two boys for cheap since September and so am here everyday.
Fast forward to now. Their wedding will be in August of this year and I'm not in it. At first I assumed it was because they were having a small wedding and therefore wouldn't really have a wedding party. But I want to say about a month or so ago she went dress shopping with all of her friends and sister (who took a train just to be there) while I babysat, so it pretty much clicked that those are her bridesmaids. I got really upset and the few I told (including my other brother) said for me to stop babysitting for them. I tried, told my brother that they needed to find somebody else and I looked for another job. But they kind of brushed it off like I never said anything and nowhere has hired me yet. I tried to ignore it. 
But now I'm not even sure if they want me to go? I watch their sons in their home, mainly in the living room and she left these two sheets of paper out here pertaining to the wedding, one was a checklist of what was needed and the other was an invite list. My name was in parenthesis, as was my bf's (whom they don't like) with a frowny face, and her brother's (who she does not get along with). Nobody else's name. It makes me think that I'm just invited out of obligation. I'm very upset about this whole thing and every time I think about it I start tearing up. It makes me not even want to go at all. 
Am I being overly sensitive and hormonal? Selfish? I'm not really sure what I should do. Talking to them wouldn't change anything. I will be 34 weeks pregnant at the time of their wedding if that means anything.