I had to change to only seeing my ob once I found out it was twins. I was devastated because I had finally found an amazing GP that I wanted to see throughout my pregnancy, although he actually turned out to be the anaesthetist for my birth so he managed to take part anyway . I wasn't even allowed to do the midwife clinic so she is quite lucky in that respect.
My pregnancy was consider higher risk because I had one placenta for the girls, although they were in seperate sacs, so had to have extra scans. My best friend who had twins 2 weeks before me, also had to go under ob only care, although her pg was 'perfect' for a twin pg - 2 placentas, no complications and made it to 38 weeks.

Best of luck to your SIL!