The bra is not your friend. The purpose of its existence is to bind and restrict her breasts.
Since you can’t rip it off completely, take your hand and slide the straps off her shoulders. Then fold the cups down so her breasts are free under her shirt.
“Now that the bra is out of the way, you’ll be able to rub her breasts and squeeze her nipples with ease, and she can still get the bra back on quickly if needed,” says Cassie Fuller, co-owner of A Touch of Flavor, a sex education company.