I opened a can of worms for my daughter

I have thought long and hard. My daughter has been asking about her older sister that lives in the same town we do. Her sister is from a previous relationship her father had before him and I met. Sadly I think that one ended with him cheating also, but that is beside the point. I have always raised the children to know they have an older sister. It's never been kept from them. Now that my daughter has gotten older she wants to get to know her sister. The mother and I don't speak though. With Facebook being the place to find people these days I set out to found her. I can't deny my daughter the chance to get to know her sister. I felt there was no harm in contacting her.

It could go either 2 ways. The children get to meet their sister or the mother doesn't allow it. Now throughout this whole time their father has told me that the mother doesn't consider my children to be her daughters siblings. After being raised with only my sisters and then finding out I had a brother and sister in Ohio I can imagine how my daughter feels. To her that is her sister. I guess maybe I am second guessing myself. I stated it was for the children. That I didn't want to discuss him at all. I just want the children to get to know each other since they live literally 20 minutes from each other. If we stay here chances are they will end up in the same school. Maybe I should have left it alone but I wanted to honor my daughters wishes. I know how much it means to her. I am just not sure what to tell her if the mother doesn't allow it. Any ideas? I know some of my readers may have some type of experience with this. Maybe even advice. Was I wrong to ask? I hate not knowing if I did the right thing. Usually I know if I did the right thing, but for some reason I am second guessing this one. I guess I am just worried on what I am going to tell my daughter if the mother doesn't allow it! ARG!