Sis Miscarriaged her baby and found out she was carrying two

This morning my sister miscarried her baby, she was 9 weeks and 5 days, she passed the child. Then 3 hours later they performed an ultrasound and advised her that she was actually carrying another baby still inside of her alive and a great heartbeat. Howevever, this second baby that is well and alive is measured to be at 6 weeks only. The doctors are spectating on if my sister has two cervixes or if the baby is just not developing the correct way. Is it possible that she could have gotten  pregnant at two different times or what? im just very confused

[Red2282]

I saw a thing on the news around christmas.  A lady in Englind had twins but they were 3 weeks appart.  Can happen very rare.  Wish your sister the best.  I will search and see if I can find anything on it and post if I find it.  

[Red2282]

http://abcnews.go.com/Health/ReproductiveHealth/pregnant-woman-pregnant-julia-grovenburg-conceives-consecutively/story?id=8656369

http://www.kfsm.com/news/kfsm-river-valley-grovenburg-babies-twins,0,2013716.story

two different cases of women having gotten pregnant weeks apart.