[TOMT] [YA Book series] American orphaned siblings sold separately into slavery, one sibling's story per book

So I've been trying to find these books for ages. I've asked various librarians, and a friend who I know read them too, but it hasn't rung bells for anyone.

I read them mid-90's, and I'm pretty sure they were at least a decade or two old then.

The basic plot:

• Set in America some time when slavery was still around. There's a family with a number of children - 5? 6? 7?, and through some sort of tragedy, the parents die. The kids have no relatives and no money, so they end up sold off in a slave auction. There's one book per child, we follow the stories individually.

• I think the first book tells this bit of the story, and then follows the eldest's journey. The eldest is a girl, maybe 17? and she's sold to a reasonably nice family to be their governess I think.

• There's a boy who's one of the middle children. He gets the worst of the lot, his owners are mean, treat him like a slave and beat him. I think he gets helped by the slave railroad and runs away to meet up with the older sister.

• The two youngest children (one boy, one girl, the girl is the youngest) get sold together to really nice people who treat them as part of the family. I think the girl feels home there, the boy being older remembers his real family and never quite settles in properly.

• I believe that at the end of the series everyone ends up back together again, whether through running away, or being bought back by a sibling.

If this rings any bells at all, however faint, I would love to hear it.