Is this your "sister in law" as in, she's your brother's wife? Or is she your husband's sister? The relationship does matter. If she is your brother's wife, your place is to take this to your brother rather than to her. He may not be seeing that she is overwhelmed and/or needs parenting lessons (alongside him!). She may figure "I left the 8-year-old in the tub at that age, what's the problem?" and that's how the baby ended up in the hospital. You don't mention him at all in this posting which is worrying if he's your brother; he is ultimately just as responsible for the kids as she is, even if she is the one present. The law will hold him responsible too, and CPS will iinvestigate him too, so he needs to get on the ball here.
If she is your husband's sister, it's your husband's place to have this talk with her, or alternatively, to approach her husband with a "We're both dads here and I want to know if you need help?...." conversation.
It's good CPS is already on the radar here. I would want to know if CPS is going to do follow-up visits at any intervals or never again? Is CPS keeping them "on the books" for extra attention for any length of time? Not sure how you ask them that without their cutting you out of their lives, though. If not, there is not much else you can do other than offer -- without accusation or "we know better" comments -- your help with whatever they need.
This is a tough one because of the distance involved. But I do not agree with those who are hassling you in their replies. You don't come across to me as butting in or as acting like you know best. It is NOT merely "parenting differently" to leave your young kids unattended for three hours inside the house while you are where you cannot see or even hear them, and it is not merely "parenting differently" to leave a nine-month-old in the tub, and so on. You know in your gut that these parents are in trouble but you just can't see them enough to know how serious it is or isn't.