She is your sister-in-law, which means that she is your brother's wife, and they live with you in the same building. I suspect that living together at the same building with the parents of one of spouses isn't a good idea. I know you're from a tradition and culture where extended family still has some presence, but still it's not a good idea. The parents will always try to intervene with the justification that they "care", and I don't think any one would like that. As for you, just try to be nice, and don't escalate things. That's all you can control. Blaming others is not a good thing within a family.