That would be the equivalent of her "water" breaking. (The "different sacs" you're talking about are the amniotic sacs.) You are talking about monochorionic twins.
It could be a leak, and the membrane can repair. She is producing amniotic fluid all along (until birth) whether or not the amniotic membrane is intact, so it can fill again even if she leaked or her "water broke." If the rupture in the membrane seals, it will be a situation in which the pregnancy can go on. Likely she is on bed rest & being monitored, and this is what they are going for. Keeping things tight until the babies are viable.
Encourage her to drink plenty of water, eat plenty of protein. To hang tight & have faith. Barbara Luke (physician) wrote a book on carrying multiples, and I bet a lot of her information is online too.
But primarily, encourage your sister because she is probably physically uncomfortable AND scared. Listen, and care.
at 1:42 PM on Sep. 2, 2013