I'm stoked that my sister-in-law is getting fat
What makes you so happy about her gaining weight?
I never liked her and hope she gets even fatter.
You shouldn't judge others based on their appearance, so it's wrong for you to hope for your sister-in-law gets fat. You should base your assessment of someone on how they act and their personality.
I'm her brother-in-law so I got every right to wish her fat. Besides, I don't like her.
I think that it can come across harsh if you state things like that. It is kind of unfair to judge others, especially based on their looks.