Pattyj, what kind of relationship do you want with your father?
Do you feel comfortable discussing his relationship with his wife with him?
How much do you want to hear about your mother from him? None, a little, what?
When you can decide what it is that you want, you can set boundaries to protect it.
If you no longer wish to discuss your mother, you can act on that, "Dad, I love you, but I'm no longer discussing my mother with you. How are YOU doing?" - You can provide suggestions, but try not to take responsibility for his happiness. That's his problem. For example, has he gone to counseling? To Al-Anon?
Again, it's much healthier for you to focus on yourself and what YOU need to do for YOU. Have YOU attended Al-Anon? Sought out counseling? If you've gone no-contact with an alcoholic parent, both may be helpful for you.
As we were departing, my dad told me not to make a big of this conversation with my siblings because it would upset my mom if it got back to her. WTF?!
If he doesn't have recovery, he might not have the best language to communicate that he would like your conversation to be confidential. I think that's a reasonable request. He feels safe with you, but has concerns that your siblings would share the information with your mother. Then, he would experience consequences? from your mother.
Again, not your problem. His. He would need a lot of work to feel comfortable directly confronting his wife. Not to control her, but to express his feelings as they truly are; Right now, it sounds like he's not ready (regardless, triangulation would be a poor flow of information to your mother - from you to sibling back to your dad - that's a lot of grapevine to pass through - it's better that it came DIRECTLY from him).
It makes sense for you to NOT pass on the information since you would be participating in triangulation.
Just my two cents. I can't speak for your father - I'm guessing on what he's feeling here.