I'll weigh in and say it's quite possible for tandem nursing to happen, even with twins and a toddler. Just make sure she nurses the twins first and gives them priority regardless. The first month it seems like a baby is always on the boob anyway, twins or otherwise. (I have a 2-month old, so I've been there very recently!) I would, if I were you, encourage her to keep an eye on the babies' weight. If that and their output is appropriate, then all is well. Breasts work on a supply and demand basis, and having the extra stimulation may actually help with her milk supply.
And don't worry about the potty-training....it's quite possible that at 2.5years, he isn't ready for it yet. When he's good and ready it'll be a cinch but I wouldn't pressure her with that with two new babies in the house. He won't go to highschool still wearing diapers. But if it makes you feel any better my son (who is perfectly normal) didn't get potty trained til he was well over 3, and then he did it in less than a week for daytime dryness. He didn't stop using Pullups at night until after he turned 6. Sorry, don't mean to get preachy. Just want you to not get worried about the PT bit.
ETA - wishing your family the best!