Look on the bright side. You protected him so well he never knew the real her.

Time to move forward and stop wasting time then. Maybe your brother views your mom differently despite the abuse. People are different. Don't be ashamed that you protected you and him. You're a good person.

Tattoo doesn’t mean approval. Ppl get tattoos as a reminder to never forget sum thing horrific.

moms are special, even if they're abusive. he'll get over it in a couple of years. a lot of people get have emotional trouble around 20 ish, but he'll keep reflecting on it and realize the truth eventually. The more you make him feel mad about it, the more he'll cling.

he'll realize one day that he shouldn't have gotten that tattoo. it's only a matter of time

Sometimes, abuse looks like wildly favoring one child over the other. The favored one may never realize that there's a problem. But the abuser always needs someone to abuse. If you had left, that would have been your little brother. You did protect him.

As someone else mentioned, some tattoos are battle scars earned, or a reminder of past trauma endured. If the tattoo was indeed, a tribute because he treasures her, then you shielded them and soon they will realize on their own the abusive behaviour.

Don't judge yourself by the actions of others. You did what you thought was right, so you're better for it.