I'd have to bite the bullet on some of the things they've said, like doubting your illnesses. I'd have to ask them why they think you are faking an illness. I'd put it nicely, explain you have no intention of coming between them and their son or grand child(ren), but neither can you accept someone assuming that you are lying.
Seeing that this is your inlaws, I'd also explain to DH it's his family and it's mostly upto him to sort this.
When we wanted to limit the amount of time we spent with my family, we always traveled to them, so we could get up and leave once we'd had enough. So we'd go to their house, or meet at a restaurant, function or a park. We'd try and time it for after lunch, but during the afternoon so we had to go home for dinner/bath/naps by a reasonable time.
I'd make a point of inviting them to every event at my kids kinder/school that had a set finish time. I sent them kinder/school photos, make sure they got birthday cards etc from my kids but all at arms length. So I couldn't be faulted, but nothing was loving/caring/sharing.
Good Luck, some family suck