Why do people keep asking about her parents? I’m sure OP would have mentioned her parents if they were alive / in contact. My mum died before I got pregnant, it’s not uncommon - do people really not think about others having different circumstances to them?
OP, we had twins with no family help - I’m stunned your SIL got three months of help, that must have made things very easy on them! But many of us don’t have any family help and we all manage and you will too if that’s what happens.
I had an emcs (recovery didn’t go brilliantly as i ended up with cellulitis in my leg and was really unwell), both twins in nicu, then one at home and one very sick in hospital, then both at home, then I had to stay in hospital with one of them for two weeks...
The good thing is that you’re not outnumbered! Two babies, two parents - no problem. Your DH will have to step up and do as much as humanly possible for the older baby (not sure if you’re breastfeeding but even if so, he can do everything else!). If you’re really concerned, see if you can arrange a nursery place for the older baby, even if it’s just a couple of sessions a week it will give you some time. Yes, without family help you will both be exhausted and it’s even tougher if you’re unwell yourself but if that’s the case your DH will have to power through - mine ended up looking after one twin at home for the two weeks where I had no choice but to stay in hospital with the other, they were still tiny and he managed. We were both first time parents with no experience so we were thrown in at the deep end but we worked it out.
Try not to place so much importance on having them there - I’m sure they’ll still be there to help anyway!
Once you have a child in school you can only travel abroad during holidays so it’s not like it has been done deliberately.
Try not to panic, you’re both experienced parents and you can do it!