Siblings Fighting

I have a 2.5 year old girl. My spouse has 2 girls that are 17 and 7 they spend 50% of their time with us. The 7 year old and 2 year old fight constantly and over everything. We have a gate blocking the 7 year olds room so we can keep the baby out but she knows how to open the gate. My spouse doesn’t think so but he always blames the youngest in every argument and it’s coming between us in a horrible way. He has absolutely no patience so he immediately starts screaming when they start fighting and it’s at the point now that I’ve been taking my daughter out of the house when everyone is there just to avoid it all. When it’s just me with them it’s hard but bearable. Yes the 2 year old grabs things and won’t share but the 7 year old is a major tattletale and will say LOUDLY whatever the 2 year old does wrong just to get her in trouble. I’m at my wits end. Up breaking up won’t help because there’s no outcome where my daughter won’t be put in this situation. She gets timeouts but they’re not working. Has anyone been in this situation??