Advice please -please answer

My husband hasn’t seen his mom in a while. They talk all the time though. My husbands sister didn’t treat him right so they cut off contact and the sister and the mother live together. So we haven’t seen my husbands mom in forever even though we talk. My husband agreed to help his mom moved since his sister is out of town. His mother’s apartment is completely destroyed. It’s infested with fleas and bedbugs. My husbands sister and her 3 children live there and his mother. His mother is moving somewhere different but letting her daughter and 3 kids live there and leaving the lease in her name and pretending she still lives there. The sister completely destroyed the apartment. It is unlivable. There’s clothes piling to the ceiling. The amount of bugs is ridiculous. The smell is completely unbearable it smells like dog and cat piss. The sister been taking a vantage of his mother for years. She is very old she is in her 70s. Is there anyway I can go about complaining about the house to landlord? I truly feel the children should not be allowed to live in the house with this condition. Children or adults should not be able to be living in this house the condition is unbearable. My husband wants to do something but we aren’t sure how to go about it or what to do first