I would definitely do something because that is not a safe situation for the baby. He is already causing lasting damage to the baby just by yelling at it. The most impressionable time in a child's life is the first 3 years-- that's when they learn attachment, safety, trust, etc. Children who grow up in abusive households have lifelong issues with attachment, trust, feeling safe, etc. and, as they grow up, they are at significantly increased risk for trouble in school, drug/alcohol abuse, sexual promiscuity/teen pregnancy, or becoming involved in abusive relationships themselves as either the victim or the abuser. Even if the baby has only experienced verbal abusive so far, that's enough to be dangerous-- and it's probably only a matter of time before the baby witnesses the physical abuse and, most likely, becomes a victim as well.

Since you have said the dog is also being abused, the dog should be removed from that situation. Animal abuse is not okay either.

Of course, your sister does not deserve to be abused either. However, as an adult, that's her decision to make. You and your mom can (and should) encourage her to get help and leave him-- but, if she absolutely won't go, that's her choice. However, she does not have the right to subject her child (or dog) to abuse-- it's against the law.

If you trust your mom, I would tell your mom everything and I would decide what you want to do. Call child protective services? Call immigration? Talk to your sister, ask if she understands the harm she is causing to her baby, and see if she is willing to get help? You have a lot of options in how you deal with this situation. But I would do something.