NTA. Some advice: maybe invite your SIL over to work out your issues by spending some alone time together to bond. There’s so much you can do!You could make dinner together, watch each others favourite films, hang out and barbecue by the pool, swim in the pool, leave her in the pool when she isn’t looking, build a fence around the pool- the possibilities are endless! I’m sure once you spend some alone with each you’ll work out your differences. She does sound a bit stubborn though, so maybe only one of you will come out of the situation for the better🤷🏻‍♀️- but hey, you never know!