Yes I agree that it is a bit more complicated than just getting your name taken off the mortgage. I would suggest that you speak to a solicitor about this to find out a bit more, before you speak to your ex's sister. If you speak to her beforehand, just say that you need to get some advice first.
I am not convinced that just because you were on the mortgage, that you now own the property. I have a feeling it will be a bit more complicated than that. Something tells me that if you were to claim the property, that the ex's family would take this to court if necessary, where it will be shown that you had not contributed to the mortgage or house for the last 20 years. There must be some case law in this area, as I can't imagine the situation is unique.
It may cost you to get legal advice, but I think you need it, as I am sure you will start to get correspondence about this, which you may not know how to deal with. i.e if you are still on the mortgage, surely the mortgage company will try to contact you.