OP,
I have a 6 yo who has lived with us for 3 years (adopted) and no, I do not "let" my DH travel alone with her to my in-laws. DH has mommy issues like the ones you describe and I know he would turn into a 7yo as soon as he got there and MIL would end up basically doing everything for DD (MIL's dream) b/c I saw it happen with the other cousins, who are now older. This would lead to more problems down the road b/c MIL has boundary issues so it's important that we keep everything stable and just the way it always is, just to keep everything clear (she's the type where "you give an inch, she takes a mile"). I never thought it'd be this way with my ILs b/c in fact, I think it's quite a caricature and old tired parody of what in-law relationship should be like, but, as my therapist has said, "Caricatures and parodies have to come from somewhere!" meaning, that is b/c it DOES happen so many times in IL relationships

so, I have over the years sadly come to the realization that t his is how things work best for us.
Good luck to you.