Blimey!
Seems like your SisIL didn't change the sheets for 3 weeks, let alone wash them or strip the bed! As for the 8 towels, were they all the same kind of towel or different, maybe 3 bath towels, 3 hand towels and 2 washrags, for example? If they were all one kind of towel, then that's a lot. And it would suggest that, for some reason, she changes towels often though not sheets. If they were different kinds of towels that makes a little more sense.
Regardless, I think she should, at least, have handled the sheets the way you requested. But as a PP (previous poster) said, she may have forgotten. Also, I do think there may be a cultural difference. And no, that doesn't mean I think other cultures are barbarians or whatever. It just means there are different ideas about how to do things in different places. As you can see, even here, there are different POVs about guests stripping the bed, etc.
Then again, perhaps you've never expected this of her on her regular visits? Then no reason to expect her to do any different when she's there alone, I'm afraid.
Personally, I don't mind guests not stripping the bed or putting towels in the laundry, etc. if they are just w/ me a night or two, as is usually the case. But if someone were in my home for 3 weeks, I would expect more from them. At least, put the towels in the laundry or fold them and leave them near the hamper.
However, I wouldn't "make a fuss," I would just make some decision. For one thing, you and DH might want to set some new ground rules with her, in general. But I agree with PPs, I would not let her stay in my home alone again, no matter what excuse I had to make.
In fact, I take it she only asked b/c she knew you and DH would be away, is that correct? If so, then perhaps it's better not to let her know when you'll be gone?