J.S.
My son did this constantly between 18 and 24 months. I nursed until 16 months. It is definitely a phase. He still does it occasionally when he is tired and needs reassurance. My advice is to calmly tell your daughter "out of mommy's shirt please" and take her hand out. Persistence over resistance! If she has a "lovey", maybe steer her towards it during those times. My son did it to everyone he felt close to, grandparents, aunts, uncles, even his little sister! They all thought it was a compliment that he felt so comfortable with them, but I know it is slightly embarrassing out in public! Good luck and remember, this too shall pass!
J.