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.