OP had it right all along.
She is NBU as they are not her parents. But her DH was also right to say yes, as they are his parents.
It will be hell, OP will resent them being in her space, with her child, her time, her everything.
But her DH will be doing what he fells he should to support his parents with his space, his child, his time, his everything.
It is best to get the rows done and dusted now, OP. That way you will both be able to support each other through the six weeks. Your DH will have to accept that you do not love his parents in the way he does, that you think of them as close strangers. You will have to accept that he loves them the same way you love yours and he is comfortable in doing this (yes, I am assuming a lot there, I know).
What you really need to get sorted, right now, is that this is a one off. They will not be coming to you every year for those six weeks. No way, not ever! They must get something more sustainable in place. Mind you, after this year I would assume that everyone will be on the same page, anyway!