I want to shit in my sister's bed.
Are you joking? Why on earth would you want to do that in your sister's bed?
I want to humiliate my sister because of her mischievous ways.
That is disgusting, it is wrong to defecate in bed. You should do that in the toilet.
But it won't be humiliating for her if I shit in the toilet.
You shouldn't try to humiliate your sister by going to the bathroom in her bed, or by any other means. If you have an issue with your sister you should talk to her about it, not do something awful to her.