Not only is this beyond disgusting, but it's disturbing that your husband hasn't read him the riot act. This is no way for him to treat his brother's house! No way for him to treat his niece! It's GROSS! And, beyond the pale. He needs to clean up after himself. He wouldn't be setting foot in my house if he pulled this, um, crap.
It's one thing to clean up after a toddler, or someone with dementia or something. I've done plenty of both. A grown adult, presumably healthy? Nope. Not in my house.
Seriously, you need to ask yourself why this is okay with your husband. Does HE want to clean up after his brother?