OP, does your DH communicate with his parents by email? If you're reluctant to have him handle this for some reason, would it work for you to write it and have him send it? I think the approach should be to leave the final decision to them, but present it in a way that makes it easy for them to opt out.
Something like: Mom and Dad, I appreciate your offer to host our rehearsal dinner, but we're at the point where something needs to be committed to and defined. There are 2 remaining issues that need addressing. First, both Lisen and I are very uncomfortable with not including her sibs and her grandmother at the dinner. They need to be invited.
Secondly, we're being asked about arrangements by out of town people who need to make travel plans. We're too close to the wedding to not have answers for them.
We are more than happy to take this off your shoulders if it's too much right now, given the busy holiday season. If you still want to host it, this is great, but you're putting me in the awkward position of having to submit a deadline for final arrangements. We're spinning our wheels too much on it right now, and I don't want this to cause a problem between us.
So please let me know. If you can invite Lisen's sibs and grandma, AND if you can get your arrangements completed by (insert date/time), this is great. Otherwise, it will be easier on everyone if we just take charge of it ourselves. Love, (his name)
Edited to fix names