To come at it from another perspective:
Would you like it if your SIL made you wash the way she wanted you to when you go to her house
Or maybe she does!

In that case, how does it make you feel? You don't want your guests to feel uncomfortable.
My golden rule of guests: I try to make them feel as comfortable as they would in their own home- and hope they appreciate it by doing the same. But even if they don't- oh well
That doesn't mean I go out and buy a bunch of stuff- my in-laws drink TONS of pop, but we're water/milk/herbal tea drinkers. We do get a 2 liter, but that's it.
Also, my feeling when I go to other's houses- I live a bit differently than other people, and I have to accept that. I don't expect others to go out of their way to change their lifestyle when I'm there- I feel it's being a good guest.
However, I don't think it would hurt to grab a container of Ivory liquid soap, easy to get, more crunchy than other stuff. Purell can be drying- we used it when I was a teacher when colds/ flu ran rampant.