Advice???

I have 3 stepkids 2 girls 1 boy and 2 bio kids… Anyways tonight one of my stepdaughters caused a fight between me and my 14 year old daughter and I’m so hurt about it. Idk how to deal with it. We never fight or anything and we always talk about how each other feels but tonight it’s totally different and I cried and she cried. After MY daughter and I started arguing my stepdaughter left the entire room and had a smirk on her face and then went an called someone and started talking about it with whoever she called. She was laughing an everything. Ughh I’m so annoyed I hate feeling like I’m a bad mom or arguing with my child cause of someone else. It hurts my heart soooo bad!!! 😢😭😭 Stepdaughters dad even calls his own daughter an instigator because she does that with her own siblings too. She isn’t happy until someone in the house is fighting or something I’m so sick of it. She’s 17 but acts like she’s 13 herself still. Idk. Am I overreacting or wtf?? Been in these children’s lives for a while now but their attitudes never fucking change. So damn disrespectful to ME too.