how to help someone with depression when dealing with my own depression?

my sister (who basically is my bff), is really depressed. i want to be there for her, but i’m currently dealing with my own depression. for the most part things are okay; we both try to push through and live our lives, but sometimes she just cracks, reaches her breaking point etc. and just breaks down and watching her break down makes me more depressed because i hate seeing my sister like this. what can i do to support her?