Helping Mother In Law

I have sex with my widowed mother-in-law, Pam. She's 64, quite good looking, slim but she's quite shy and does not want to go and find a man for a relationship so she gets sex from me. My wife is OK with it as she knows that it's just sex. It's usually one or twice a month when she will call my wife to ask for me to come over and fix something. She's not that active in bed, straight missionary usually but I can sometimes persuade her to get on top or let me doggy her. No blow jobs unfortunately!