Use your words. Tell your parents you are done with having a front row seat to their addictions and will no longer be coming around until they've taken steps to get healthy. Sounds cruel, right? You need to realize that any other decision only allows them to continue harming themselves and puts them at a very real risk for an early, and possibly tragic, death.
Then talk to your grandparents. You can't make them believe you; you can only make them aware. If they choose not to believe you and cut you off because you are setting healthy boundaries for yourself, then you need to take another long hard look at your grandparents as they could be part of the problem.
Decide what is or is not tolerable for you. Set your boundaries. Enforce them, even when they try to bully you otherwise.
And please, don't try to do this alone. Seek out the services of a competent, qualified therapist in your area. If money is an issue, check with local churches and civic organizations for free or low-cost services.