M.K.
You (or anyone else in the family) do not need a good relationship with 'mom' to suggest anything! The bottom line is this- that child comes first. The fact that she peed all over your house (not to mention 'mom' or bil didn't offer to clean it up (how rude!)) is enough for you to say "Look, you take her in and get her checked out or she doesn't come or stay over anymore! That's it- end of story! I have a very irresponsible brother in law (and his soon to be ex wife) as well and sometimes getting upset with them and making them take their children to the hospital is the only way to do it
Stand up for this child, you may be the only one who does, and do it soon! My neice ended up with a UTI that ran up her kidney's to the point she was puking and my MIL had to force her parents to take her to the doctor.