What should I do??

What would yall do? So my husbands older sister was telling me how awful their younger sister is still treating her oldest kid (he is 2 with disabilities). She barely feeds him. He drinks mostly milk and he’s 2! She LEAVES HIM by himself at the house when her, her bf and her youngest (1 years old) go and do shit. She left him alone on Christmas while they went to my husbands aunts house. His older sister said he looks SKINNY and sick. He’s supposed to have glasses and a wheelchair and leg braces and has none of that. I truly think she’s only keeping him around for the check she’s getting. Her and her bf call him “Deadbody” and have everyone else call him that too. He’s starting to hit himself now bc he’s so frustrated and obviously can’t communicate his feelings bc he’s disabled! And now that she’s pregnant with a GIRL, i truly feel like she’s gunna mistreat him even more once she’s here. Would yall call cps or leave it alone? His older sister said she talked to some of the family about it and they told her “just talk to her and if it doesn’t change then call cps” but I’m like??? That’s ridiculous that you have to tell someone to stop abusing their own child! His older sister said it’s REALLY BAD. Like the neglect is heartbreaking

Edit: the only reason I’m hesitant is bc the older sister is the only one who has witnessed this and she’s scared to call bc she doesn’t want the younger sister to know it was her. She has gotten cps called on her 3x already and they keep closing her cases and she feels like they won’t do anything

Please do not come for me. I truly am trying to do my part with the facts that are being given to me. Keep in mind, we don’t see that little sister ever. Only the older sister does.

On the phone with CPS now! Thank you to everyone who commented

UPDATE: I apologize for not being on to provide the proper update but cps was called and unfortunately closed the case…. Now, that poor baby is in the hospital undergoing surgery because he broke his femur and my SNL “doesn’t know how”. I’m so heartbroken…