@burcidi-- It's possible if irritable uterus occurs early in the pregnancy. For the most part though, it tends to happen in the second trimester when the baby is bigger. Particularly women pregnant with twins experience this because the uterus has a hard time adapting.
Your sister will have this risk in her next pregnancy too. If she becomes pregnant again, she must discuss this with her doctor to take the necessary precautions. She will probably be put on medication and told to rest. Staying as far as possible from stress is a must.