16 months and always hitting

My boyfriend has a 17 month old little girl and she’s the best thing that could ever come into my life. I’m close with her mom and her moms side of the family, as well as my boyfriends side of the family. Well, her moms side of the family has a ton of little kids. Two in particular are VERY mean to her. They will hit her and push her and bite her, but their parents don’t say anything. We will tell them to stop but they keep doing it, none of the kids have been disciplined at all and now my boyfriends daughter is starting to hit us, and other kids. Earlier she threw an ipad at the cat and hit him in the head with it. I told her No in a firm voice, but every time you tell her no she goes into a hysterical fit. I’m trying to be easy on her but I don’t have any kids of my own and i’m only 22. I don’t know how to even TRY to get her to stop. Every time we talk to her moms side of the family they laugh it off and say it’s no big deal. I don’t want her growing up and being aggressive because they don’t discipline their kids! We definitely don’t want her going to school when she’s older and biting other kids because she sees it’s okay. We need advice on how to deal with it and anything could help