5 month old

How big is/was your baby at 5 months? Weight & length. My SIL baby will be 5 months on the 26th and she barely hit 12 lbs (she was 6 lbs 14 oz, 19 in. at birth). She hasn't got much longer since she was born, she's maybe 22 inches. Everything about her is still so tiny, she wears 0-3 month clothes but some things are even baggy on her. Now I know some babies are just small but her mom wasn't feeding her enough in the first few months. If you knew her you'd understand why I'm concerned about the baby. Also, about 3 weeks ago she contracted salmonella in her blood. I know this could very possibly happen by accident but it could also happen from carelessness. Im not sure if I should leave the situation alone. The mother (my SIL) hits her other 2 kids (ages 4 and 6) with a plastic hanger as "discipline". I feel like this has gone too far honestly and I'm close to reporting her to CPS. The 6 year old is from a previous relationship and my SIL boyfriend hates him. He's always yelling at him and treating him horribly. I've never witnessed any physical abuse but in my opinion that poor little boy is emotionally traumatized. I'm pretty sure I know what I need to do but I'd like everyone's opinions on the matter. Also sorry for it being all over the place, I kept remembering more things.