I have another thread about this in AIBU and have concluded our 2-bed house is too small to accommodate his parents and adult SIL for a month! No tickets booked yet but rough dates in mind.
I like my own space and privacy. I need quiet time in eve to re-charge. My DS has smaller bedroom (used to be guestroom). He's up for day at 5am and still has night-feed(s) so I go to bed early. When teething he wakes multiple times a night and can take ages to settle.
When DH first informed me his family would be visiting, he wouldn't really discuss it. In his culture you welcome family with open arms, bend over backwards to be hospitable, even if it means overcrowding/inconvenience. He assumed I would accept this and shut down my attempts to air my concerns 
I find it very stressful having guests for a week let alone a month. I can't relax. Our house is small and full of toys/baby things, and our bedroom is tiny so we store most of our clothes in cupboards in other rooms. When guests come I'm constantly wading through piles of clutter and tripping over things. Fine for a week but not a month!
I don't want to spend a month gritting my teeth and hating it. I want to enjoy time with my PILs and SIL and for them to feel comfortable too. The thought of everyone squashed in is so claustrophobic! I want us to be a big happy family and I don't think I can achieve that if we're all on top of each other!
I feel we need to get them a holiday-let or AirB&B. I'm happy to host them in our house from breakfast-early eve each day, have all meals together etc but I need some downtime at night. I need a quiet house to sleep, time to take a shower and get dressed etc before they come over in morning.
However I know DH wants the full experience of 'living as a family'. He is at work all day so won't see them as much. I feel he could always go to their holiday-let if he wants to stay up late with them after work, while DS and I get some sleep. I will be entertaining them all day, taking them on trips etc. They are lovely people and very considerate, helpful, but I need some space!
How do I tell DH we can't accommodate them here, without it turning into a massive row?