AITAH IF I DEFENDED MY SISTER FROM HER "BULLY"?

Hello, I(18F) has a younger sister (7yo), who has now started her school after the summer vacations and she goes to school by a public transport and along with her other children of her school go to. That includes- her classmate(sashi), sashi's brother(13-14yo), a boy from another school (dk his age but he's also around 13-15 yo) and a 5th standard girl. My sister is in 1st standard and has transferred to a new school this year, and then we have arranged a public transport because everyone in family is busy and their time don't match with her school time.

When she first started going, there's was no problem and she told us that shashi and her brother are very nice and friendly towards her. Yesterday she came home crying, like bawling and then when we asked she told that shahi's brother was making fun of her, he said that she looks like a cow because she makes 2 pigtails and that she is a crybaby, he also held her hand and said that he'll throw her in the drainage. The other boy told her to chop off some of her hair, and bring doll so he can show her magic. And these boys together make fun of my sister daily since the vacation ended and told her to go die. I was astonished and felt so bad for her because I was also once a victim like this when i was in school. My mother said to my sister that myb they're just joking and it's alright but I felt really uneasy. I told her to tell it to the driver, she said he also laughs along.

Today when her driver came to pick her up in the morning, i told that boy that it's not funny, he said that's funny if she's laughing, i told him that I'll report him to the principal and he's gonna get suspended i told him his face is funnier than his jokes and that if he finds it to funny then say all that to his sister too. He made a face and said alr whatever. The other guy, he doesn't go to my sister's school and also he gets picked up at last so I didn't met him, And I told the driver to tell that other boy to act sanely or it won't be good. To which the driver replied that- "when these boys make jokes, she also laughs along and say more things in back." Bro wth does he mean?? Ofc she's gonna take a stand a say back, what do you want her to do?? Stay quiet??

Later my mother said that I did wrong and should've talked it out not shout like that, i told her they deserved it and she said no, they were joking and also that my sister is naughty too. Ik my sister is naughty, I'm also fighting with her always but just because she's naughty that doesn't mean she's allowed to get bullied. I always shout at her because she's taking my things but yesterday I felt like crying when I heard about what she said. My mother said that shashi is in her class and also that her brother sometimes help her too and now she's gonna feel left out and they'll never help her. I understand her concern but this doesn't justify their behaviour. Nowadays I'm also seeing random News about how 13-14yo boys have been SAing little girls and now my head is so full of thoughts that what if something happens to my sister just because I snapped at her bully. What if they bully her even more?? AITAH???

I'm worried about what if something happens to her, what if those boys do anything wrong to her or bully her even more, its been 1 hr since I confronted them and since then I've been so anxious, I've been a victim of bullying too and as a result I've many mental health issues, I don't want her to go through that too. I also wish someone had my back when I went thru such bullying but no one did and I felt so shitty and depressed ,so now i don't want my sis to feel shitty too.