Well, now that he's 27 you can make it in several ways, or just leave it open if you prefer. He's five years older than Karafirelight, so:
1) He could have dropped her in a basket in front of the door if you want Kara to have been left at the orphanage right after her birth. This would have been four years before the Great Death.
2) He could have done it personally, say their parents died two years before the Great Death, when Kara was two and Joel was seven. According to the scale, a pony that age would be more or less like an eleven year old human, so even if the "old mare" was really a Grandma (before the Great Death any age was possible ^__^) I don't think she would necessarily have insisted on him staying or something. I only said she would have insisted if he had been a one year old baby carrying an a few days old baby @__@.
3) You could say that the parents' death happened right before the beginning of the Great Death, when there were still adult and old ponies around in the orphanage. Kara would have been four by then (about seven for a human?), though, and she probably would have some clear memories of her brother. I don't know if you wanted that to be so.
4) You could say it happened right during the Great Death (and that this was the reason why their folks died), so the "old mare" in the orphanage wasn't really that old..she was just an eight or nine year old pony like Joel, so yeah, they were taking only crawling babies ^__^. Er...do baby ponies crawl? @__@ (blushes).
IN SHORT: I think there are plenty of possibilities. Whatever floats your boat ^___^ . Or you can leave it open if you prefer, not saying exactly how old Kara was when she was taken to the orphanage. It was the not-so-great-a-difference between Joel's and Kara's ages that made the history seem a little odd to me, but now that this difference has been broadened I really see no problems.
So...I was gonna give you my approval

, but it looks it's no longer necessary

(points at two approvals) Anyway, he's approved. Have fun!