The un-spayed females being inside your house is probably the cause of the male's marking the spots inside.
The spots most people can't even smell, stand out like big red flags to other dogs. Is there a reason your in-laws
have a unspayed dog? The first dog was also un-spayed and she probanbly left her own spots as well. It's no
wonder your male dog is confused.

Probably need professional cleaning to remove the spots & the odors that remain in the house. Because people
can't smell them doesn't mean they aren't there. Good luck fixing the problem.