Dog urinates while visiting parents, not at own home

Question
My brother and sister-in-law recently adopted a 3 year old lab/Pyrenees female that weighs 90lbs. The dog was 10lbs overweight when they first got her, but has since returned to a healthy weight. Previously, the dog lived in a home with multiple pets and was put up for adoption because she asked for a lot of attention--the shelter recommended her to a home with no other dogs. She is friendly to all people and dogs. She's actually quite interested in meeting other dogs and she totally ignores cats. Here is the problem: While she is totally house trained at her home, when she visits my in-laws she urinates  inside. This has happened twice. My in-laws have a 13 year old mutt, also female. The dogs do not get along that well because the 13 year old female generally does not like other dogs. The dogs growl at each other, but no serious altercations have happened. Why is the 3 year old urinating on the carpet? Is it a territorial issue? Is she nervous around this other dog?

Any advice you have is much appreciated!

Answer
It could be a number of things.  Is she truly housebroken at home?  She may be unable to generalize home from new place (she urinates in the yard at home, but is unsure where to go in the other house) or she may be reacting to the other dog.  It depends a bit on how successful she is at home (is she going outside on gravel or grass and is this option unavailable at the in-laws).  Also if she is given an opportunity to go before arriving at the in-laws.  Also (lots of possibilities) is she actually urinating or is it a  little bit of excited or scared urine?  If she is excited or stressed and a little urine is coming out then they need to work on some calming exercises around the other dog before going into the house.  Click and Treat for appropriate behavior and letting her out often to pee.  If she is actually urinating (like they do outside) then she may not realize she is in a house (substrate to similar to what she normally goes on).  They may also need to do a mini re-train  to work on house breaking her in a different environment.
I hope this helps