When the the annals of codependency are published, let this be the inscription at the beginning of the book. Dude, that is not healthy relationship behavior. Like LMTL, the "it is only right." caught my interest as well. Your mental work here is interesting. You are sending the message that the behavior you're planning is self-sabotaging and self-destructive, but we find out that your narrative is that you are only doing this because you are so deeply virtuous and selfless and committed to doing what's right.... for your sister.

Part of the twisted fun of a codependent, in contrast to a truly pure addict, is that they find a way to call their illness virtue. I, personally, am an expert at it, and I applaud your skills. Honestly, though, I think it's far more likely, though less romantic, that you just want to get wasted to numb your feelings which is the opposite of T, and if you can find an excuse to run further from your feelings by abandoning T altogether, even better.

I don't see any reason, except being a chicken (I say this in the nicest possible way, I am not mocking you), for you not to go. Except the one that Snuffle articulated so beautifully in her post. If you can construct a choice where you can truly believe it's T versus your sister, well then of course you will choose your sister. Because you are so virtuous and all that.

Seriously, though, go to your appointment and deal with this like an adult. Talk to your T about what you are planning to do and why. I don't think the issue is so much whether you go on a bender (although, as others have pointed out, it has the potential for one or both of you to become really hurt), but to explore what's underneath it.