Stayin' Alive
Well, if the adoptive dad was an orphan, that makes things pretty simple — just have them meet, draw parallels between their situation and his, and guilt him into it.
If you need more solid reasoning for it, maybe they wanted a son already, and after meeting her younger brother and learning he had a sister, decided to adopt them both? Heck, maybe the dad even had a older sister who died in the Orphanage of Fear, and sees the older sister and her brother in a similar light to how he used to be with his own sister?
Nemo enim fere saltat sobrius, nisi forte insanit Deviantart.
My teacher's a panda
I guess the easy solution would be that the couple has always wanted a big family, but something had happened since the birth of their second daughter and they can no longer have any children biologically. Maybe there were complications during the last birth, and decided that if she were to go through it again, it would kill her. So instead, they waited until they decided they were financially capable of taking care of additional children and then went to adopt.
^^this, very much so.
Doodles
Could I please ask you some (more help?). I keep trying to come up with something, but I can't.
The story is set at an steampunkish world. When the protagonist was seven , she acidentaly caused the fire that killed her parents and her older sister (by either having the brilliant idea of cooking breakfast by herself, or letting a lantern fall, I don't know which one of the two is better).
She ended up at an orphanage, stayed there for two years and then got Happily Adopted alongsides her little by a traveling merchant and his wife, a teacher, with two daughters in their teens. But I can't figure out why would they adopt her and her little brother.
The protagonist's adoptive father is an orphan himself, and was raised at an Orphanage of Fear until he got adopted.
Help?
Ah, summer, what power you have to make us suffer and like it. ~Russel Baker