Lost:
Sorry for your situation.
Well, what is worse: The drinking or the possible infidelity? As far as the possible cheating, I say a person can most usually go with their gut on that and maybe that's a wrong thing to say, but as my mom used to say, "A woman knows". Of COURSE she would deny it. Just as addicts deny,deny,deny. Denial is part of the disease. Denial is a very common thing. I am not trying to stir up suspicion if you have already forgiven her, but since you've brought it up, I'd guess it's far from resolved and you still have doubts, even four years later.
But let's say if she did cheat that night. Has she done it since and/or did it stop? Can you forgive it? Because she may or may not have cheated, but if she did, did she regret it and did she stop that behavior. What do you think?
As for the drinking: big problem. And yes, it's progressive. What can start out as just going out and partying with sis every now and then can grow into a regular habit of drinking and before you know it full on addiction. I know I'm not telling you anything new.
Anvil nailed it as usual.
No matter what SHE did or didn't do; does or doesn't do, YOU can still DO what you need to do to have peace, joy and fulfillment in life; with or without her. Empower yourself and don't feel one bit guilty for pursuing your own happiness.
I tend to look at it this way. We are each a separate entity, even though married.
Will that be easy? No. But worth it.