I'm really not the right person to ask, because I'm incredibly bad at the TV show canon anyway.
But my impression from reading fics has always been that there aren't that many years between them. After all, their parents' marriage turned rocky pretty soon. Does it sound all that likely that Sam and Ella would have Lois, start fighting, make up after ten years and have Lucy? (And then start fighting, but not break up, all over again?)
I'd say I'd expect them to be about two years apart, as that seems to be the most common interval for parents having children. But if it fits your story, I don't see why you can't make Lucy three, four or possibly five years younger than Lois. Not more than that, however, I think.
Ann