Idealist

does your sister feel comfortable speaking about her feelings and the depression she suffers?
Just letting her talk about it may be so helpful to her

I think I have mentioned before that I have both a husband and son who periodically go into depressive phases, but who have very different needs when this occurs. My husband does not want to talk and just wants to be left alone, whereas my son wants to talk and not have me say anything, just listen

they both benefit from natural supplements for the depression, although they prefer different ones, and both reacted very negatively to prescription anti depressants..............does your sister take anything for the depression?

Your just being "there" for your sister may well be the best therapy for her....just knowing that she can talk to you and that you arent going to judge her or offer cliched advice or tell her to "be positive" or " get over it" any of the other pat and meaningless phrases that people so often use ......... so often just being able to verbalize our feelings helps us to get them into perspective, and having a listener rather than an advice giver is so beneficial!

I have also learned that there doesnt always have to be a reason for depression....for some people depression simply is......and that especially is when a caring friend or relative just being there with quiet support is so meaningful

sooooo
having said all that
basically I am saying that it seems to me you are already helping her so very much