No thank-you from unknown parents of daughters staying

We are in loco parentis to two nieces from abroad who are at boarding school here, 6th form stage. They usually come to us for at least one weekend per half term, plus half terms, though at different schools so not always at the same time.

One of them has several times asked to bring a friend for the weekend, one who has no family near and would otherwise have to go to an ‘official’ host.

All the girls have always been very pleasant, polite and ‘easy’, and apart from the need to stock up the fridge! it’s never been a problem at all, they’re old enough to pretty much look after themselves.

However, not once have I had any sort of thank you from the parents of the extra girl. Not a phone call, not a text or an email. They must know they come to us, since the girls have to get their permission.

There will be the 2 + 1 this weekend, Friday to Sunday. I’m very happy that niece in question feels comfortable to bring a friend, but I don’t think I’m BU - am I? - to feel irked with parents who don’t seem to think any sort of thanks are called for.