Bloody marvellous! It all kicked off last night and I've spent most of tonight in tears.
Following the excellent advice on here, I agreed that the best way forward was to discuss the problem opening with MIL and SFIL. So I called them. And left a message asking if I could talk to them both about going down to the French house and please could they call me back to discuss it.
MIL called me back and said SFIL was even more cross at hearing my message. Her exact words to me were "But don't worry. He'll calm down. We just need to talk it through with him."
At dinner a few days later, I suggested to MIL that it would be a good time to have a chat. But she was very clear that she didn't want me to raise the subject as her sister was joining us and she didn't want a row in front of her.
Over the following weeks, I suggested to MIL on several occasions that we needed to talk to SFIL. But every time she said "It's OK. You are going on holiday to our house in August. I'll sort it out."
Only she didn't. Last week she told me they were driving down to La Rochelle and, thinking everything was resolved, I asked whether they had room in the car to take a travel cot down as it would be easier than us taking it on our flight. "Yes, fine" she says "No problem. Drop it off at ours". I specifically asked whether she'd discussed the matter with SFIL but she was very vague and just said "He'll be fine about it."
So yesterday afternoon I did. As nobody was home, I left it on the doorstep.
Last night I had a call from SFIL yelling down the phone about how upset and cross he was to discover this travel cot on the doorstep. He'd asked MIL who'd obviously told him and he'd gone nuclear. I immediately apologised and told him I could understand why he was upset as he'd heard nothing more about the matter since early Feb but that I had called and left a message (which he acknowledged he'd heard) but MIL had told me not to raise it again and she would sort it out with him.
He started going on about how he didn't want anybody else to use the house as he'd put a lot of hard work into the place. I queried why this had changed from previous occasions. His explanation was that rental guests paid a lot of money and that went towards the bills. He said that, although MIL had lots of money and paid some of the bills, he didn't have her income although he wished he did. He was thinking of selling the place and splitting the proceeds between them so "at least then I'll have some capital behind me". I offered to pay rent but he said that wasn't the point.
I told him we wouldn't go and we'd make alternative accommodation plans as we couldn't cancel our flights. This was obviously a bad move, as he had no idea we'd booked flights and had thought we intended to drive down!
We ended the call and agreed to talk more later as we were both getting quite cross.
I'm really upset now and worried about what will happen. The reason we asked to go to their house is because money is tight so, if I'm honest, we can't really afford to book other accommodation. I'll also have to give my cousin her flight money back as none of this is her fault. 
And I'm pretty bloody cross at MIL for being such a wet blanket.
I mean, what did she think would happen when she loaded a travel cot in the car as they were setting off? I've spoken to her tonight and she said she was too scared to raise it with him because he'd get cross.
What a mess! I feel like I've been dropped into the middle of their relationship difficulties. MIL says that she's sick of him never wanting to do anything for her family. He says he's sick of her family coming first and that she does too much for us. Personally, I'm sick of the lot of them and wished we'd just gone caravanning in the West Country!
Please be gentle as I'm running out of tissues....