here comes the matron of honor: the reply
When you talked to your sister at the time of your wedding, she said she had no interest in your relationship better. That's pretty much a "Get Out of the Jail" . You can use it as an excuse or simply to explain why she didn't make the right . Before you say no, however, look at your past with your sister. If up and coming through for her now could be a healing thing, then I think you should at least doing it. If you do decide to go with it, however, decide in how much time and money you are willing to spend and then stick to it.
If you know that agreeing will only mean you'll end up getting manipulated and taken of, then feel to say no, but make sure it doesn't sound like 'an eye for an eye'. You can tell your sister you are for her, can't wait to meet the guy, and look to the wedding. You can add that you would be happy to help organize a shower, for instance. But explain that since you know your and you know what the duties of matron of honor involve, she needs to find someone who really has the time to do them .