I also have twin girls 1 year now... one of my girls is a bottom scooter and the other is a crawler. I am in PT for the scooter, they said it was due to low muscle tone in her core, she would not even roll over. we have been in PT for about two months now. She is rolling over like a champ but still scooting, and cruising now on furniture. They told me at PT that as soon as she walks we are done.
(you have to get on the floor for these exercises ) You could try to get them in a four point position (on knees and hands) and put your hands on their bellies to secure them and rock them back and fourth, putting toys in front of them to get them to move their arms forward, also when she is side sitting I hold her foot that she likes to lead with (the right one) under her left leg and eventually she goes onto all four by herself and just kind of hangs out there, but at least it adds strength to her arms!!
Good luck, you may want to go for an evaluation soon, the earlier you start the easier it is!