He sounds enmeshed with SD which is common.
SD is an adult now and she needs to learn some boundary setting etc and realize that she is not responsible for him and not able to fix him.
She is calling you to fix it and now you feel obligated - you are also not responsible for him nor can you fix him. This sense of obligation and guilt (SD) will cause you to ruminate and worry like mad.
My friend - you cannot make it OK for everyone. Can I recommend a really good book?
I Don't Have To Make Everything All BetterWorkshop - US: What it means to be in the “FOG”That sense of obligation was very reminiscent of my own childhood.