No, it's not right at all. I hope you don't get mad at me saying this, but it's a very cowardly thing to do. She should be "cleaning" her own messes, not have you do the dirty work.
I guess you did the right thing by telling your friend about the break up, as you don't want him just hanging there. You could always talk with him more, to help him feel a little bit better about the situation.
Next time, if possible, try not to let your sister put you in that situation where you have to talk to someone for her. That is wrong, and the ultimate sad part is later on in life, she won't have you to do it for her. Break ups are going to be very hard on her, and she has to learn that it's part of dating.
It's obviously hard for her to do it, so maybe talk to her without getting too upset to let her know that it is her responsibility to tell the other person that she is no longer interested in dating anymore. She owes it to the other person, as they both were part of the relationship.



