Is it inappropriate for in-laws to stay with ex-wife?

My husband's ex-wife lives a few miles away from us. His dad & stepmother are coming in for a visit and are staying **with his ex-wife**. This is the first time they have come to visit since my husband and I have been married and have our own place (we've been together 4 years).

We have 50/50 custody, so they could have easily planned a trip for time when stepdaughter is with us. Or they could stay with us and make arrangements to visit with sd with us or with her mom. They could also afford a hotel.

When mother-in-law comes in, she always gets a hotel. She does visit with ex-wife, and I am fine with that. I have no problem with the in-laws being friendly with her (in fact I think it is a great thing!), but staying in her home when they are supposedly coming to visit US is crossing a boundary, in my mind.

I think they are doing it because she has a luxurious home, while ours is modest (though they've never even seen it!).

When ex-wife's mom comes in to town, she doesn't even phone my husband to say hi.