PL, I hate to say it, but you D needs to tell your H how she feels instead of dumping on you and bringing you down. My grandmother is telling me the same things, and I don't even want to talk to her anymore, because I just don't need to hear it.
I'm glad you are telling your SIL. The secrecy thing doesn't work for me either. Keeping it a secret almost makes it feel as if its your fault or something, like you have something to hide. Bearing this burden is hard enough without having to pretend.
The LORD is near to all who call on him, to all who call on him in truth." (Psalm 145:18)