my sister posts that she has nobody and ignores me

I don't understand.
My sister is posting about how nobody is ever there for her and that everyone makes her feel like a burden. 
Yet, I message her and ask her how she's doing. I tell her she's not a burden and that she can talk to me. 
But she'll fucking ignore me and then post stuff like that🤷‍♀️ i guess double jeopardy you can't blame me twice. 
When she's happy, she'll message me and talk all the time. But when gets depressed she acts like this and pushes me away and complains about how nobody cares about her. Ok. I don't know what to do.