Lol I don't understand why people are getting confused and thinking Anna Correa is Molly's sister.
Anna Correa- Pregnant woman who was probably in a relationship with Dr. Logan. Logan couldn't help Anna due to the laws of Crawford. Anna never stated she had a sister in Crawford.
Now if you decide to get the code for the locker in season 1 episode 4 Lee will watch a tape. The tape reveals that Molly is from Crawford and having relations with Dr. Logan, so she can get meds for her sister. Molly never said her sister was pregnant.
After coming back to the school Molly reappears to give Lee the battery. Depending on what you choose Lee could be angry at Molly for lying to him, or he can reason with her.
If you choose to be angry with Molly about being from Crawford, Molly will reveal that she went back to get a picture of her sister. She also told Lee that her sister is Diabetic that's why she had relations with Dr. Logan. She wanted to get pills for her sister. Logan didn't give her any pills that's why she went crazy on him as a zombie. She also showed Lee a picture of her sister but didn't tell the name. So why are people saying Anna is Molly's sister Lol. They are two different people. The tape with Molly asking for the pills for her sister was made days before the tapes with Anna Correa.
Anna wanted pills to prevent Oberson from knowing she's pregnant perhaps with Logan's baby since they were in a relationship. Logan couldn't do anything about it.
Molly on the other hand wanted pills for her diabetic sister. She made a deal with Logan so he could give her the pills. Having a sexual relationship with Logan did not work. Molly thinks her sister is dead, but could possibly be in Carver's camp, or Tavia's. Tavia did say they have a community of people of all ages. I don't think it was Anna who was killed by the river in season 2 episode 1. It could be just models. Anna and Molly's sister could be in Carver's or Tavia's camp.
Here is a picture of Molly's diabetic sister
Hope this post helped everyone to not be confused about who Molly's sister is :)