How to comfort you're sister ?

My sister has been having really strange dreams lately. She use to have them all the time when she was a kid but she got them maybe once or twice a year now it's every night. She said they seem so real. Last night she dreamed that she went to school there were all the schools in the county all connected together they were different countries she kept getting lost she didn't have her phone she kept going around to different ones. She said it's so real she can hear people talking. She's home schooled or she was she graduated this year though I don't see why she would have these dreams or what is causing them. She doesn't like scary movies she don't watch them at all. She's pregnant but even when she wasn't she still has them. This morning she was crying in her sleep she said they not bad dreams but some are scary. Nothing bad has ever happened to her. I was laying here holding her she fell asleep on me it's too early for us to get out of bed. How else can I comfort her ? Why is she having these dreams ?