There is something disturbing about the way you juxtapose your family being a massive help with the children etc while she cannot walk much or take on cooking.
No, this is a daft statement and you know it. OP is talking specifically in the context of something that is supposed to be a treat for her, not about life in general.
This thread is so bizarre. So many people saying that a woman must dilute a relationship with her own family.
I would find it totally bizarre to invite my ILs on a family weekend away, and so would my ILs. I value the time I spend just with my own parents, siblings (and partners and kids) and my nuclear family, because we interact in a totally different way in that context than if there are others there.
And exactly the same is true of the time we spend with DH's family. That time as family is precious and it would undermine that relationship and the value of that time if it was constantly diluted by inviting everyone. DH and his folks interact in a totally different way when it's just us (and his sister and her family) than when there are others around that they don't know as well.
I would honestly be shocked if someone said I couldn't spend time with my mother and father and my siblings without inviting my in laws because it would be 'rude'. I've been on holidays with just my family and (less frequently because they're not really holiday people) DH's family.
The question doesn't really arise - my family and DH's live on different continents and don't meet that often; they get on well enough when they do. But they are not each other's families any more than my two grandmothers were family to each other.
OP - there are some seriously batshit people on this thread. If you were having a barbecue or a huge fancy dinner for your birthday then I'd think you were being U not to invite your MIL. But a weekend away with your own family as a birthday treat for you is a totally different thing - and TBH I'd present it to your MIL as your parents and siblings treating you to a weekend away if there is a risk she'll feel offended.
I think the dual celebration is the right way to go.