Thanks for your insight. I had not thought of it before but you are right, sister 2 is grieving the loss of the family unit that we used to have. I know grief takes time and goes in stages.

I worry that the sadness will make her mental health worse. I feel bad that she is sad.  I worry about holidays/birthdays, etc.  It’s like she is the child of divorced parents; she will spend half a holiday with me and half with sister 1.

I think it helped her feel better just to air her feelings to me, and get them off her chest. But I worry that it will keep coming up again and again.