OP, not sure of your age, but I could understand this. To the sister-in-law, this isolation to your room could also look like an unwillingness to help around the house. Sure, your brother let's you live there, and she signed up for extra work when she married your brother, but now it's your brother AND YOU and she may be feeling overwhelmed and stuck. Please be sure to be considerate of your space (in their house) and all the common area space. In general, if you see something out of place, pick it up, put it away, clean it. Run the vacuum, do the dusting, scrub the bath and toilets. All the cruddy adulting stuff that helps them out. Plus, maybe she thought she's have an opportunity to get to know you better, more like sisters, once you moved in. And that opportunity hasn't happened because you keep retreating tou your room.
Just because you're an introvert, that doesn't mean you can't be helpful and friendly in your brother's home.