Mother "spanks" my infant sibling

To avoid being specific in case my parents search for it, my sibling is not even one yet. They are around crawling age, so it doesn't take a rocket scientist to come to the conclusion that an infant doesn't know right from wrong; despite the fact that they can do no wrong regardless. It's not a simple pat on the diaper either. She aims for skin. I have told her several times not to do this because of that reason and she just plays it off as a joke/laughs about it. My sibling is very active and hyper, so they squirm a lot and wants things they can't have. My parents are the type of people to prioritize respect and obedience over ANYTHING else and I remember being spanked since I was a small child myself. The rest of my siblings (there are a lot of us) are not/were not exempt from it either. I have never seen it this early on, though. How psychotic do you have to be to hit your child, let alone an INFANT? Whenever I bring it up, she's always like "____ understands me when I tell them 'no,' so they know what they're doing." I cannot believe I have to tell a grown woman who's had several children (as the very lovely man upstairs has "said to do" /s) that you CANNOT hit a baby because they don't UNDERSTAND. It's not funny and I never laugh at it, so what is this stupid ass show she keeps putting on? I swear she acts like a child herself. Also, my dad has done it a couple of times, but I refuse to try and tell him to do anything as it only results in me getting abused too, and while that's bad within itself, it doesn't bother me as much as the mental toll of conflict within itself does.

TLDR: My mom hits my infant sibling as "discipline" and I'm getting frustrated.