I don't want to babysit my sister in law's infant child
Her child needs you, it is just a wee little kid. Is there a reason why you can't babysit?
I was once babysitting that child and he wouldn't stop crying for hours.
Imagine if you were that crying baby, then imagine having to care for yourself. That might help.
Babies shouldn't cry it must be abused
It's perfectly normal for babies to cry. You should offer to babysit and if you have any issues doing it you could always call her to ask questions.