Urgent!! Need help

My husband's sister is someone I avoid because she's toxic. I leave her messages unopened too. Our family knows I don't like her. Recently my husband's brother created a group with that toxic sister, me, my husband and himself. My husband had mistakenly asked his brother to add me in the group. So since they made that group, my husband's toxic sister got a chance to send double-meaning messages in the group for me to see, and has been sending tons of her pictures too. I haven't opened that group because I avoid interacting with her.
I want to know if it's okay to just reply nicely ONCE to her pictures, wish them all well and then exit the group? I don't want to leave the messages unopened because she'll keep sending them. I don't even want to mute the group as she won't know I muted it. So is it okay to just exit the group after replying nicely once? What would it show?