Oh God 10! Jk but kinda not. I have two girls in that age range SD and DD, there may well be tears and tantrums because that's the way they are. Mine always fight and over the stupidest stuff. Like SD trying to force DD to use the hillbillies of the loaf of bread to make a sandwich when SD wouldn't dare use them! (Just happened last night) but I agree with finding a new place and talking out how the parenting will be done. I think that you should let your SO know exactly how you feel about that and that you want to be respected and listened too. Whether you are very involved or not. When I moved in with my SO we had a "family meeting" where we expressed our feelings to our girls and explained in nice terms that in this house he was Dad and I was Mom, they could call us whatever they felt comfortable calling us but the basic jist is that we together would be parenting all of them. Our words are to be respected whether the bioparent was there or not. We weren't rude about it and said it in the nicest sweetest way we could. I wasn't extremely tough at first I eased into making changes to how things were done (when needed) around the house and had the edicts come from my SO when they were to my SD. Now as time has passed we've had many ups and downs, mostly related to outside forces trying to manipulate my SD into hating me and my DDs, but we really function as a family and most people have no idea that all our children are not biologically both of ours. Just be very open to your partner especially when you are not happy with something that is happening your opinion should matter.

I'll leave you with this quote from comedian Louis CK it made me laugh.

“Boys are hard to raise, man. All my sisters have boys and I just feel for them because it’s really hard. Girls are just as hard to raise but on a whole other level. They just — they’re different. Here’s the difference, to me, between boys and girls: Boys f—k things up. Girls are f—ked up.”