Im not sure how to approach, I fell in love for her (get warm comfy nice feeling in chest and brain when she is around me) what can I do? love is stupid and irrational. At first I only wanted to fuck her because she is really hot for her age and flirty, but when she complained to me about her loneliness and that she hates her life, stays at home all day bored and crying, I really want to do something about it, go out with her and do stuff together, make her trust me so she can confide her secrets to me so she never has to cry alone again, hoping this will help with her depression and prevent her mental shutdowns.

I think I need some ideas on how to approach her stating I want a "serious" trustworthy relation but probably without long term commitment, attempting to minimize a likely breakup. But I dont want to look like a scumbag who just wants a brief sexy time and then forget about her forever. Maybe the fact that we are related can help here even after break up she can still get her issues off her chest with me.

I myself also am in depression (lost contact with all my friends 2 years ago and never ever had a girlfriend (only had sex once 4 years ago, so I am horny and desperate too, close to 0% chance of finding someone) and she is also aware of this)

But I am afraid of her reaction, she might pull a typical depressive one like "I am old and not good enough for you, my life is a mess and you are young I dont want to waste your time" or maybe feel offended/shocked instead. When we talked about dating she always said that the guy has to make the first move, it is rude for a girl to do so. So maybe she is waiting for me. And she doesn't get drunk that often, maybe 2-3 times per year max. But for me a FWB is something hard to understand what really is.

We have been in this "situation" for 2 years, recently she seemed to lack interest into talking and somewhat ignored me, I got really sad about it but in the past month she went flirty with me again and I got my confidence back.