My niece is sick so I want to give her medicine without her mother's consent.
Why would you do it behind her back?
I'm just tired of having to take care of her while her mother is at work, she wouldn't do anything and just eat and shit and cry.
I think that you should at least tell her that you are giving her child medication. I think that that is the right thing to do.
I'm going to make her suffer so I can get some peace and quiet.
That's a good way to ensure that you never see your niece again. If she tells her mom what you've done then you may lose privileges to be with her, because you should always let a parent know when you give their children medication.