Taking my first sip of the water

[SH_]

Let your w and s work on their own relationship. It is not for you to get involved or try to have any influence on that.
And as Altair says don't over evaluate the reasons behind your kids doing what they are doing. Truth is it could be a million different things that influence the behaviors. Sure your r with the w is causing strain for them but is is not the sole reason and you can't fix that.
G my friend stay focused here.
You need to do what is right here for you first.
I agree that validating both of them is good, but don't over step your w ability and authority with s. Don't worry that s will be upset with you because you don't step in.
It is a tough spot to be in with an in house s, but there is much info out there about co parenting that can provide insight for you.

Keep learning and stay focused on fixing you. You need to put on your oxygen mask before you can help everyone else.