A little background to start...
My fiance and I had planned to pay for our entire wedding event, from start to finish. His family graciously offered us the money to cover the caterer (after asking what that cost was), which is great and obviously a huge portion of the expenses. They've already give us money towards it (i.e. paid the deposit) and I have every reason to believe they will keep their word and pay for the remainder of the catering bill. My family has no extra money--when I go out to lunch with my mom, I usually buy.
In addition to the money issue, I'm sure if I gave my family some guidance as to where to call, they could plan a nice rehearsal dinner. However, I'm pretty sure his family has never planned any kind of event ever, and would have no idea where to even start. They are much....simpler....people than my fiance and I.
Enough background, on to the question. We are fully capable of throwing our own rehearsal dinner, and happy to do so. But we don't want to step on any toes. Should we ask his parents if they want to "host" it, essentially meaning paying for it? How do we phrase that in a way that says "We're only asking because it's tradition and didn't want to short you of an experience you were looking forward to hosting, if that's the case". Or how do we ask both families if they want to "host" it together, which could mean we pay for it, but they just essentially plan how it's going to go on that day (i know we'll have some speeches and things that they could plan for), and do the RSVPs. Can we ask them that in a polite way?
If we do host the whole thing ourselves (again, totally fine with that!) is there any way we can have someone else dealing with the RSVPs?
Currently, I'm planning on sending a separate invite to those people invited to the rehearsal dinner (immediate family and the bridal party, of course with all significant others invited). For the record, it's about 30 people.