Am I in the wrong

My brother asked me to pick up my nephew from daycare. I asked if he had a booster seat ready, cause sometimes my brother and my nephews moms side, will let him ride without one. He told me it would be fine without it and that he even asked his mom and she said it was fine too since it’s just a 7-9 minute drive.

I told him he won’t be able to sit in my vehicle without one. Now I’m being accused of being a bad aunt, saying I just dont want to pick him up. That I’m “extra”.