Ask her when your alone with her, explain your reason... older they are better your chance - whatever age. I did family also & it happens more than you think, a few days later spent the night with her/them that was start of a wonderful thing - my gran had 41/2 yrs of it + often worn out even when she was in a nursing home like the rest she/they wanted more. Lets talk about it....
Both make a great couple, granny still putting out with wanting cock at her elder age.... no harm in that. Just enjoy what she offers & more the merrier.