<human>: A few weeks ago a friend of a friend told my best friend that he got a "new batch" off my brother.

My best friend told me but he wasn't 100% if it was my brother.

Skip to a couple of days later and my brother bursts into my bedroom saying he is opening up a business with his ex girlfriends dad. I think nothing of it, so what. A little adventure... Then it sort of doesn't sit right. Him and his ex aren't exactly friendly anymore so why is her Dad wanting a business with my brother? 

Skip to today. I'm asked to baby sit short notice by my brother and I accept. I go over and then they leave to different parties. 

My girlfriend comes over who says she can smell something weird. I go up to the faint smell of weed and the sound of an induction fan. I try to open up the door to this room and it's locked. So I boost the mrs up onto my shoulder to peak through the glass above the door (luckily it wasn't blocked off) she says she can see what appears to be a black canvas/plastic looking wardrobe with a small fan on the ground and a box to next to it. Outside the door there is a bucket and watering jug. 

It all hits me that my friend was right and my brother is growing/dealing this shit. This is made worse by the fact he is growing it in his family home a couple of doors down from where his son sleeps. Then the business thing sets in: he must be doing this to "clean money". Her Dad smokes enough of the shit why not grow it?! He's a shady cunt.

I'm really lost as of what to do. My girlfriend says to tell my mom but it'll really hurt her... I don't know what to do. 

All i can think about is my nephew in that house and the possibility of the police kicking the door in could mean he would be put into care. 

I don't know what the fuck to do. It's 5am here and It's rolling around my brain. 

Please help me, Reddit.

Can you condense the text into a few key points?
<bot>: My brother grows weed in the house his son (aged 4) lives in. I don't know what to do.