I don’t know what to do

Let me start off by saying sorry if this is in the wrong group, I just don’t know where to post it to get advice. Ok so my SIL has a place of her own but she is over my house all the time so much so that she spends the night every night and she slowly started bringing her stuff over without really asking about moving in. She has “lived” with us for about 6-8 months now. She is 27 years old, but she has some emotional issues and my husband says that it is best for her not to be by herself, like she is the type of person that will always need to live with someone because when she is alone she will trash her place to the point where it could be on the show hoarders, I know because I had to help clean her place multiple times. In general she is a great person and helps out with the kids but she never cleans and is such a dirty person but if you didn’t know her on a personal level you would never know.

Ok so my issue is that she has 2 cats at her house that I don’t think she ever checks on. I ask her about when is the last time she went home to check on them and make sure that litter boxes are cleaned and that they have food and water. She either tells me that she does whenever she used to go to school and got off later or she doesn’t answer and changes the subject. I have asked my husband and he says he doesn’t know that why don’t I ask her so I told him the exact same thing that I have told you guys. We have the spare keys to her house so yesterday I was able to go over to her house while she was not around to peak in and check on everything. Well as soon as I opened the door one of her cats is dead on the floor right by the entrance and you could tell she was there dead for a while. I have sinus issues so my nose is stuffed up most of the time and thank God because that smell most have been horrible. I was in complete shock and disgust. I didn’t even go in and try to look for the other cat but I knew he had to be dead too because he is the type that will greet you at the door and constantly meow. So my issue is is how do I bring this up and what should I do? I went behind her back and I was being sneaky but my gut told me to check. I feel like I violated her privacy but I was worried about the welfare of her animals and had to see how they were doing and if they were ok like she said. So she has lied because she hasn’t been checking on them and they have been dead for months, you can tell. The reason I didn’t check on them earlier is because I have a pretty busy life myself and didn’t really have the time. That is why I was asking her about her cats and just stupidly trusted her when she said she would check in on them. What should I do I really need some advice. Sorry this is so long I just had to tell someone and I don’t really have anyone to talk to.

EDIT: I just found this out yesterday and my husband is away on a trip but he gets back tomorrow, she is good when others are around just not by herself. Like I said I am absolutely disgusted and in shock and no she should never own an animal ever again. I am currently looking up my states laws for animal abuse/neglect because I’m not sure what they are and what measures to take.

Edit #2: I didn’t check on them sooner because I just got access to her keys. Also she was going to school and told me that after classes she was going over there to check on them. She has never lied before or at least I never caught her in a lie and she does take care of herself like she showers puts on makeup and functions in day to day life. I even told her to bring the cats over to our house so that way we could watch them and take care of them but she said no. I went over there as soon as I got keys to her place. The worst I was expecting was that litter boxes needed to be changed or food refilled since she said that she was taking care of them but obviously not