Something is really wrong.can anyone tell me where these inside mouth blisters came from?

My brother and his wife came back from their honeymoon which was in the tropical islands. I believe that they may have given their daughters something in the mouth. They have broken, bleeding blisters inside and outside of their mouths. They both can't eat, drink, urinate, etc. My sister in law has taken them both to the hospital here in VT and they keep saying that it's hand foot and mouth disease but they have no sores on their hands or feet. Is it possible that my brother and his wife brought something back over with them and have spread it to their daughters? Please help! They got sick 7 days ago and they aren't getting any better. Also, would you recommend keeping these 2 kids away from other children? because they keep bringing the kids outside to play, shop, etc. I told them to stay away from my 2 kids. Any help is appreciated!