<human>: So I kind of just need perspective and advice here.

My sister has recently gone through her first big break up (they had a very toxic relationship as far as I know) and is learning to come to terms with it.

She has recently started talking to my girlfriend on the phone, and has started to ask her for advice and generally trying to get closer to her. Now this I feel is a little awkward for me as I like to keep my relationships and family seperate, but the problem is that my sister wants it to be a secret from me, as in, she went and spoke to my girlfriend without telling me or asking permission from me.

My girlfriend is the type to help anyone, even if it inconveniences her. She has told me about my sister wanting to hide this from me and thankfully has kept me posted on the situation, and I do believe she has the best intentions, but it seems like my sister is bothering her (she hasn't outright said this, but I can tell.) 

My sister tells her that she feels like she can speak to nobody else and calls my girlfriend crying when I'm out of the house. She feels like she can't speak to anybody else and so has turned to her, telling her things like she cut herself (which she has done before, she suffers from depression) well before she told me about it.

Me and my girlfriend both feel EXTREMELY uncomfortable about the situation. On the one hand, she is being quite "clingy" and talks to my girlfriend in a seemingly fine way after any serious discussion. She is also doing this behind my back which I don't appreciate at all. Knowing my sister as well as I do, I can say she can come across as a little attention seeking and immature, so I'm not sure if this is temporary or not, but on the other hand, how can my girlfriend turn away my sister when she calls her crying on the phone?

I really just need advice or guidance here, please.

Condense the text into a few key points
<bot>: Sister starts talking to my girlfriend behind my back about serious issues that can't be ignored, but it's putting a strain on our relationship. Help?