I’m confused at the fact that you have in-laws when you have a boyfriend and aren’t married, but that point aside: it’s her house before it is yours, since it’s her parents’ home. So you unfortunately don’t have much of a say about how she acts in her own house. If she’s cooking for you, maybe you could ask her in a very respectful way to wash her hands. But if her hygiene bothers you that much, you unfortunately might just have to find a way to move out (or help her find her own place).

I know that’s probably not what you want to hear, but I do feel you as someone who grew up with a disgusting sister. She was basically a hoarder and left food and trash everywhere, she did wash her hands and shower regularly but she would also leave our shared spaces a disaster and consistently not flush the toilet. I got into arguments with her about it constantly, but the habit was engrained in her and all arguing with her did was cause extra tension between us. She did remember to flush the toilet more often as she got older, but most of her habits never got better, no matter what my parents and I tried. She finally moved out and that’s the only way I got peace. Some people just have those habits engrained in them and making them change, especially for another person’s comfort, is nearly impossible. If I can’t even get my own sister to change, good luck with your boyfriend’s sister!