I would assume that the original purpose of this meal train was because people know that parents who have a sick baby in the hospital don't have much time to cook. I would argue that having a sick baby at home might be just as time consuming if not more so. You don't have anyone else's help, no nurses, no doctors, not a single opportunity to be "off" for a single moment. Yes, that's just part of being a parent, and of course they aren't entitled to meals but I can certainly understand continuing the meals until the original cut off date, especially since people had already planned on providing them.
OP, I do understand how this confusion could have occurred, and it's not a totally unreasonable thought that you had that it might have been cancelled. I do think that since you voluntarily signed up, that you should fulfill the commitment, as it appears you have decided to do.
In the future though, I would caution against volunteering to do things for this SIL if you really don't think she needs it or doesn't deserve it or whatever. These things are completely voluntary, and it's only going to add to your resentment of your SIL if you do things for her when your heart isn't in it. Just decline in the future...in my experience you don't even need to actually say no, you just don't sign up.