Ever since my mother-in-law caught me nude after a shower, I started flirting with her.
When I enter her house and she's home, there's always a hug waiting. I sometimes push against her a little harder; a little longer just until a breast is completely compressed and I can feel her nipple (winter visits are fun).
During a party, I'll meander towards and put my arm around her back as we talk to others. My hand eventually sidles up the side of her breast. She has never displayed any discomfort and tends to think such things are innocent and cute when her son-in-law gets tipsy.