This advice is as weak as the third brewing of a teabag (NO do not tell a single digit child it's their responsibility to stand up to their parent NO) but unfortunately I can't do better. Angelina Jolie couldn't keep her MIL from doing this to her kid: what hope have ordinary people?

I sincerely doubt this is the only way in which Other Parent utterly flouts Kid's bodily autonomy, wishes, and consent ("I'm going to ask till you say yes" sent a CHILL down my spine) but unfortunately this sort of bad parenting is within parameters for our culture. I wish I could think of a way short of a psychic blast to make OP stop.