As a "babysitter" should you be able to refuse doing diapers?

So my Brother in law's son is 4 and his school(he's in a headstart program) has just started doing 3 day weeks so they needed someone to watch him on those days he's off.

I'm a stay at home mom and I have a 6 month old so it just worked out that I could watch him.

Well I know some kids just have a late start on potty training but they havent even started. So obviously he's still in diapers and idk if you've changed a 4 year olds diaper but their craps are massive and on top of when I open the diaper to change him he grabs his poop covered wiener and plays with the poop..I have been a daycare worker before and in my opinion 4 is too old for diapers and is old enough to know better than to play with their poop..

Also..my daughter is a stage 5 clinger so kangaroo care is a big thing in our house so I'm constantly having to take her out of her carrier and take the carrier off about 6 times throughout the day to change him. And with touching poop it grosses me out with him touching everything in my home because I am a bit of a germaphobe.

I talked to my husband and basically he's told me to deal with it..but I'm done...I either dont want to watch him or he needs to get potty trained, I'll even help get him there! I've helped potty train over a dozen kids.

Am I just being whiney..? Should I not talk to my BIL and SIL?

I would like to add, I'm not saying I wont change him, I just dont want to have to change his diapers forever.