Who's the real "useless spoiled brat" here, mom?

Agree to take your sister on the condition that you use your mother's car in case lack of bathroom access leads to messy issues. My bet is she'd drop the subject.

Well that's pretty shitty.

Like your pun

Well your mom had a really crappy attitude. Wait till she walks in on you crying in the fecal position, then she'll have to rethink words.

You should buy adult diapers and fill them up as you help your sister on her mission.

Flavored Jell-O is just that AMAZING! Who's with me on that?

Hard to say. I cannot formally form an opinion until I try unflavored Jell-O.

I shall wait patiently for you to do such research.

That seriously sucks :(

tbh the second part of your FML is totally irrelevant. you don't have to explain yourself for not doing this ridiculous task

No, the second part is totally relevant. Not only does OP's mom want him to drive his sister across the city (why can't mom do it?), but she doesn't care that he has dysentery and probably should not be driving.

If one of her children is spoiled, who does she think spoiled them?

I would personally think it's your sister's behaviour that meets that description but either way it's generally the parents who do the spoiling...

shouldn't you be in the hospital and on an iv and antibiotics

Well that definitely sucks, hope you get better soon.