Should I be upset about this?

So my sister has always been closer to my mom then I have and they have more like a BFF friendship and me and my mom make things work for the sake of my son because we have always butted heads. Well recently my mom planned a trip to Florida for her and my sister which in turn hurt my feelings A LOT! They both think I am the one being over dramatic about it but I feel like they should have at least asked me, especially my mom pretty much pointing out her favorite on that one. Me being a mom would never ask one child over another to go on vacation, it doesn't seem right. What do you think? My mom I guess assumed I couldn't find a sitter and said she didn't think I'd have the money but I then found out my sister who is 21 and can barely afford her bills is getting the free trip. Don't get me wrong, I don't expect anything free and would have made things work to go as I am a stay at home mom and would love a break but I feel like as a mother she crossed the line of respect and decency. She told my aunt she feels bad and knows that she should have asked me but hasn't said a single word to me about it and only knows im hurt because I told my sister I was through a text, who in turn ran and showed my mother. But I felt bringing it up to my mom would just put a dent in our already damaged relationship.