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I can only give you some perspective from the sil side as I am in same situation as your sil. So maybe you can get an idea what she might be feeling. See, my in laws live with me for about 10yrs. My elder bil (Dh's brother) and family live not too far. my bil and gang keep dropping in (to see his parents) at times when all I want to do is kick of my shoes and veg out. Or I might be helping DD with homework or what not. But when bil and cosis come by unexpectedly I am also expected to sit and chat, make tea etc. I adore my bil and cosis, yet part of me is pissed that I can't call my own shots in my own home cuz of my in laws. My mil will ask them to stay for dinner when i never planned etc and if this happened 2 times a week I'd be bit upset and no longer enjoy their visit. I actually would love to spend time with bil/cosis alone but always in laws are there!
Probably your sil might be feeling the same no matter how nice you are. How about asking your parents to your house for a change and give her break. Not sure if that will mend the bad vibes with sil but it it's a good start.