Reading between the lines, I think the OP’s problem with this is - she hasn’t made a plan yet for her 40th, but even if she doesn’t go on the trip for FILs 70th, the very fact it is happening the week before limits her options and she’s grumpy that because she’s not got organised first, whatever she does will have to fit round this trip for FIL, not be all about her and what she wants to do.
so she could go on the FIL trip for a week then have another week just her and dh somewhere else in either north or South America, realistically she’s not going to Asia or Australia - she’s got to fit into his plan. Which would be fine but does mean she’s not the first priority.
if the OP doesn’t go on the trip, if her dh does go, she’s unlikely to get 2 weeks away with him that go over her birthday as that would mean he has to take 3 weeks off work in a block, and how likely is he to say yes to getting off a long haul flight from California just to get on another plane with the OP to Thailand or wherever she’s picked for her birthday trip?
I don’t think that the problem is FIL wanting a trip, it’s just that the OP now has to fit her plans around the FIL getting in first with his plans and she doesn’t already have the excuse of saying “I can’t make it, we’ve booked to go to Hong Kong the week after.” Because she’s not organised anything yet.
OP it’s ok to privately upset that whatever you do, the fact this trip is happening the week before your birthday is going to cause logistical issues that limit you, but you didn’t get your plans firmed up first so now you’ve got to just fit in.
(if it helps, my 40th was supposed to be happening on a family holiday to the Netherlands the with a meal booked at Le Gavroche for grown up party the weekend after we got back- instead covid lockdown happened and I had lasagna made by the dcs washed down with some nice wine which made the next morning’s queue for the supermarket even more unpleasant.)