I have to agree with other posters that you don't sound as close to your DB as perhaps you think. I love my DBs to bits but I wouldn't say we're close as friends are - and I can well imagine him turning to local friends before me (and telling one member of the family is always tantamount to telling them all). Accept that he hasn't confided in you and don't expect more than he's willing to give. You have no evidence that his partner is behind that decision, other than knowing that she has a poor relationship with her mum which could be for any number of excellent reasons. Your present resentment is understandable but don't stack all the evidence up so it amounts to an injustice against you and your DB - he doesn't need protecting and obviously does need space.
I really wouldn't read too much into the eloping thing. It's hard on your side of the family but you can't have one side without the other. The photographs, skyping and 'free from' food show that she would obviously like to be friendly. As many have said, she could well have any number of problems that could make all this a bigger deal than you can imagine, especially if her own mum has been a source of stress.
It's potentially quite damaging to speculate that your DB has been forbidden from seeing his family. You simply don't know that anything close to this has happened and it does seem divisive to suggest it, given what they're going through and the effort SIL has managed to make. If you hear that being speculated by other family members, I would knock it on the head sharpish or you will lose end up losing them. Incidentally, I'm in NI with brothers in England...in your circumstances I would have been on the next plane over to meet their wives. Maybe she feels you weren't there at the beginning of her trauma and now she just wants people she knows.
If you're close to your brother, why haven't you asked him directly what's wrong with the baby instead of festering about it?
I was seriously ill for over a year post having my DD. It was bedlam. If someone said they were coming for a weekend, even staying elsewhere, we couldn't have coped. The pressure to entertain them, not just for a couple of hours but for days, would have been impossible. That's not personal, we wouldn't have felt able to see anyone and we wouldn't have been in a position to give a damn about their expectations. Here's how you do it. You say that you'll be in the city for a weekend doing something else but could easily find half an hour if they fancied saying hello. Maximum availability, minimum effort for them.