Have you clearly given signals that you are not interested? Like tried avoiding her, being standoffish, make it clear that you are not up for flirting?
If you talk to your gf about it, it will upset her, unnecessarily so if it isn't the case. If you ask her mother about her agenda, she will most likely deny it once she realizes you can't be fooled. Just really take a stand, try to not be alone with her, if she makes a joke with a clear innuendo, you can say "I don't think that's appropriate".
Edit: I just read your last post. It's clear that she is unhappy in life (which is the case of many 'cougars'), but it is not normal to spend that much time with your gfs mum. I understand that you are consoling her and that's that, but you need to stop it. You can't fix her life, she needs to do that for herself. She can divorce the no-good husband, but decisions like that are major and not really something a young guy like you can 'provoke'. Most wives talking about divorcing their husbands can do so for 10 years without actually doing it. She needs a therapist and as long as you sit with her for 'support', you're just helping her ignore her problems. Stop hanging out with her mother alone altogether - be with your gf whenever you're at her house.