I #closed a girl with her 67 yr old grandma yesterday...so far she has flaked though.
I just opened as normal but made sure to come from the frame that I was asking the grandma's permission to talk to her in an old skool English gentleman style. You have to be incredibley humble in your approach, I even bowed slightly as I shook hands with granny. (Once I opened a dad/daughter set and I even called him sir).
You need to charm the mom/granny eg. "Ah, now I know why your daughter is so beautiful! She obviously gets her looks from you. I bet you taught her all she needs to know
" etc. But obviously don't over do it as it looks kiss-ass or like you're taking the piss. When you go for the number don't ask the target, ask her parent
instead to show respect. Eg: "I would like to take your daughter for a coffee sometime if thats ok with you."
But again, be humble but try not to sound like you're having a laugh.
Play it out kind of like a BF would ask his GF's parents for her hand in marriage. Its fun for everyone. That's basically how I do it.