Intercourse with your grandmother might be easier to achieve than intercourse with your mother because there's more of an age separation and that type of family sex might be considered less taboo.
Reminds me of a 72 yr widow I had an affair with, she looked more like 50, often liked to play out the "spank me Daddy, I'm a bad girl' scenario. I didn't ask any questions, the results were as good as in this vid.