Visiting My Sister and Her Sick Child??

She already offered to do another weekend, if he wasn't feeling better. (Meaning, she is willing to make other plans.) I seriously doubt she would be offended, if you ask to reschedule. She said you still CAN come, not that you have to. I think she's being polite. I'm willing to bet she'd be relieved, if you asked to reschedule.

Just tell her you are a little worried about someone getting sick and you hate to reschedule, but you think it's for the best.

Just call her and tell her that while you are disappointed, you prefer to reschedule to next weekend just to be sure everyone is well.

Just reschedule. She's probably hoping that you will, so that she doesn't have to worry with readying for the visit while tending to her ill son.

And if you think it will hurt her feelings, just put it off on planning. You need to have a "decision" by today, not wait until to Friday to know if you are leaving on Friday. I would HATE going into a weekend where I woke up on Friday not knowing if I was packing to go out of town for the weekend that day or not. Maybe that's just me, but I would want it to be a decided thing already.
Just reschedule.
No hard feelings.

Tell her that you'll reschedule so she can focus on making sure her little one is feeling better and getting some rest herself (since taking care of a sick child is so stressful on a mama).

Tell her that you prefer to reschedule. She's been caring for a sick child, which is stressful, plus he's not feeling 100%. You think they'd enjoy your visit more at another time, and you think it wold be better all around. Plus, if your kids have any germs, you don't want her little one to get sick again from his weakened immune system. Position it as primarily being courteous to her and her family.

You shouldn't feel bad, this is your sister; she does understand that is why she even said something. Don't go, and just reschedule, and don't feel bad, you have no reason too....

reschedule! and be thankful you only live 2 hrs away so you can make it work.
diarrhea doesn't just last 24 hours usually, it can last days and days. Plus the little one won't be up for much playing after being sick for a day or more!

Let her take care of her baby and keep yours healthy!

4 month old babies can get dehydrated SO easily. A bug that is crappy for a 2 year old can be DANGEROUS for a 4 month old. Why take the chance?!

I wouldn't feel bad about rescheduling. I would just say that since there is no way of knowing whether it's the stomach flu, that you would just prefer to reschedule rather than risk your whole family getting sick.

You could say it sounds like maybe rotovirus and would rather keep the kids away until you know it is gone from their systems. I don't know many people that would argue with the Rotovirus card.

Diarrhea can last a lot longer than 24 hours. I would reschedule. Your sister will understand, which is why she called to tell you in the first place. She's probably hoping you'll reschedule.

Say this,

"Hey sis, I think we are going to try and reschedule. It must be crazy trying to handle a sick kid and get your house ready for guests. Besides, I don't want my kids wearing little Johnny out and possibly making him worse, and it won't be any fun for him if he doesn't feel well."

I'd reschedule . That bug goes around and around. It's nasty. Don't mess with it.

I'd reschedule. We had friends who invited us over one New Year's Eve after just having a bug that their whole family had had. They also invited another couple who they had seen just before the bug hit their family. The second invited family had 2 of the 4 family members start puking while we were all there. The original family had 3 kids have a relapse while we were there. And long story short: my entire family ended up with a raucous stomach bug two days later. I doubt your sister's feelings will be hurt, and you will all enjoy the trip much more if everyone is fully recovered.