DNA test without parental consent??

just_me
It's Christmas and all of my family is in town but no one address the "elephant" in the room. My brother and his ex-girlfriend are very unstable (classic Dr. Phil thesis couple). When he first met her she claimed to be pregnant by him...despite already being 5 months pregnant. My brother ignores her lie and played along with it and "raised" my niece now 6 years old as his own. Myself, mom and step father buys her school clothes, xmas, birthday etc. I babysit her and play dolls, take her to get mini mani & pedi's. I never deny her as family or confront her crazy mother for thinking everyone believed her original lie concerning paternity. 
Well now there's my nephew. He is 4 years old and I feel 99% sure that he is not my brothers son. My brother has concerns too but won't have a test bc of pride. I love my nephew but I want to do an at home paternity test...I really want to know. My entire family will make comments about my nephew not being blood but won't confront his mother. She is bipolar and has some other mental health conditions. So they play along with her foolishness. I do EVERYTHING for her kids and I think she owes me the truth. My brother is a deadbeat and I feel like she keeps my family around bc she doesn't have to be financially responsible for her children.  She only use them for food stamps, housing , free health care, income tax refunds etc. 
I'd never stop caring for my niece and nephew...but curiosity is eating away at my soul. I won't do anything malicious with the results. I'd pray I can keep them to myself. 
Is it wrong to perform the test without my brother or the mother knowing?