I'm going to defend my sister instead of my boyfriend when he's venting to me
Why have you decided not to defend your boyfriend?
Well, my sister has emotional issues that I have to put up with when she comes begging for help.
Be careful with not taking your SO's side when it comes to these disagreements. Often times if I am being honest, it is better to just agree with them in public and then be honest when it is just the both of you. It's very important to support your SO.
We won't last long anyway.
Usually people are expected to take the side of their so and look out for them. This could cause problems in your relationship.