Absent,
It's always hard to know how much to talk about, how much not to, and when.
Is it necessary for YOU to bring it up first? Have you considered maybe just letting them bring it up, if it's a topic they are interested in talking about and comfortable talking about....and not bringing it up if they don't?
I am currently trying to practice that. My MIL has asked after my substance-abusing family members, and the response I feel is best is, "I love them, but they are going through some things right now that make it impossible for me to be around them. I'm not comfortable with their substance abuse or how it makes them act, and hope they find a way out of their problems soon. They can't be happy this way."
And that's it.
This is, after all, YOUR recovery, not theirs. It doesn't always have to involve total candor with everyone else who may be connected with the situation. (what is your motivation for telling them? really ponder that and be brutally honest with yourself) In many situations, people are forced into uncomfortable spaces when we try to force them to be part of our recovery process. And just the fact that anything you tell your aunt/uncle seems to get back to your folks tells me that they are not exactly confidential, impartial listeners.
And if the topic comes up, think about your own way (like the above) that is 100% objective, honest, and non-judgmental. They'll respect you more, and if the truth DOES get back to your parents, it won't be anything you're ashamed of. Your parents' relationship with each other is not your business; judgment doesn't do anything but keep you in the fray.
Enjoy their love and support while you're there. Live in the moment and let them talk about what's important to them. Explore new things, have fun, be a part of their life. Your recovery doesn't have to be the focal point of your whole visit, as much as it may be top-of-mind for you. Detachment is getting on with the rest of the joyful things in your life -- maybe this will be a good time to practice it.
Just my two cents' worth.....good luck, Absent!
GL