If Meredith and Lexie really are 13 years apart in age, then maybe Molly came along a few years after his marriage to Susan and then Lexie came along 5 or so years later. Wasn't Meredith really young when they divorced? Say she was 5 or 6. Then he meets Susan and they have Molly. Meredith would be about 8 years older than her maybe. Then 4 or 5 years later Lexie is born, giving the 13 year age difference. I'm just guessing at the numbers but it's feasible.
ETA: According to Wikipedia (not saying it's right) Molly is the youngest sister and Lexie is older. So the writers are definitely taking creative license with the actor's real ages. If Lexie is a few years younger than Meredith, they sure aren't doing Ellen Pompeo any favors as you can clearly see the age differences in their faces. I guess all the years of partying are supposed to have aged her beyond her years looks-wise. And Molly sure did look older than Lexie but then again we haven't seen them side by side yet (if ever).
Okay, it wasn't easy but I managed to find out Mandy Siegfried (Molly) was born in 1972! So the writers have definitely screwed up the timeline for these sisters. Come on, would it really have been so difficult to cast according to age. Seriously?