We are more than just friends. More of brothers. Promised never to date any other's sister.
But she comes calling, I don't want my friend to look at in a negative way.
I need to get off this hook

If you really like her and think you could be happy together....go to your buddy and ask for his blessing.

* Maybe do not date. Just think of it as being friends going someplace together and have fun as friends. At the end of the "outing" just a short hug is ok! No big deal.
* Have a fun time with friends

A number one most important thing to do, is recognize who is responsible for what.

YOU are responsible for each and every choice that YOU make.

The fact that the girl is making overtures has zero to do with whether or not you are going to honor your word to her brother.

Since you wouldn't be asking if you didn't WANT to go with your friends' sister, it's time for you to step up to the plate and directly talk to him about it. And, if you decide to ditch your agreement with him, without his approval, that will be entirely on you as well. No matter what the girl does or doesn't do, you will be held responsible for your part in whatever happens.

As long as you accept that, you should be able to decide entirely for yourself, what to do.

What a crazy promise....
Max and Igor have said it nicely....
Chat with him...any reasonable guy would understand....

If you really like her, then go after what you want as long as you have good intentions for her. Approach her brother and tell him honestly about your intentions. My first bf was my brother's bestfriend and I admired his courage to ask permission if he could court me, he was brave enough to face my family and open up his feelings for me. Don't underestimate your friend and your friendship, I'm sure he would listen and understand you if you show respect to him. Don't let your brotherhood be a hindrance to get the desire of your heart. Give him respect, he will then respect and trust you and her sister as two mature individuals who have minds of their own, choices and decisions to make.

But if you don't want to be hooked up with your brother's sister, then in a nice way, honestly tell her to stop calling you , that you don't have feelings for her nor any intentions other than being just friends. And also tell her you don't want his brother to put other implications in her communication with you, for it might ruin your friendship.