Be sure to have them agree to Google's privacy policy first, because otherwise you'll be agreeing to it on their behalf.

They already have Google accounts so they should be covered from privacy perspective, but thank you for your concern.

Are they aware that everything they say to you now falls under that privacy policy?

I can understand using these for situations where you're only going to be around a language for a limited amount of time (business trip, vacation, etc). But presumably your in-laws are going to be around for a while, so what's the rationale for not learning to speak their language?

Not the OP but trying to learn another language so that you can speak it with people you see maybe twice a year seems a little silly. I would agree with you if OP lived with their inlaws or something, though.

I see my inlaws twice a year or less, still learning the language (or at least trying).

Because we have limited hours in the day?

I’ve been trying for 10 years and still haven’t made any meaningful progress; I’ve always struggled with language, even in immersive environments for several months. TBH I do prioritize spending the few hours I have during evenings/weekends with my children.