I'm sure I will be berated as its MIL, but would you expect someone that uses your toilet to flush and wash their hands after use, and also wonder why a women hasn't used toilet paper? MIL is British, barely been abroad, so its not a cultural thing that I'm aware of. We are renovating, so living in a tiny caravan- so even by staying outside when she uses the loo, its very obvious she hasn't flushed or washed hands 
Thoughts?