he is that little boy, parents gone, sister gone.

Yes! Great idea! Well, not great in terms of saddling poor Ianto with more emotional angst, like he didn't have enough angst already, LOL, but it explains Ianto's reaction in that scene so well. Plus, explains his need to take care of others, if, as a child, he felt that he should take care of his younger sister, who then died at a young age. Would be especially angsty if they lost her to a long illness, and then his mom had to be hospitalized at that psychiatric hospital in FOOTR because she went mad from grief and exhaustion... Um, plot bunny free to a good home? :p