I am so sorry - I know how frustrating it is to have a loved one who is doing something you know is wrong. My only advice for you is to maybe print some articles about the health issues of non-spayed dogs and give them to her.

I also think the following statistics might help:

In order for every dog born to have a home, every human being would have to adopt (or own) 5 dogs. So in a family with two parents and two kids - they would have to have 20 dogs to prevent dogs from being put to sleep due to lack of homes. Kind of makes you think - doesn't it?