Should I charge my Nephew for 2 rooms?

[Ibrahim Yamini]

Me and my wife are having a disagreement.  We currently live in a 4 bedroom 2800 sq ft house and my nephew, who will be 21 this summer lives with us.

After he graduated high school, I told him that he would have to go to work or get a job and he would have to pay rent.

I charged him $300 a month which is waaaaay below market value.

Anyway, he’s studying to be a notary and he wanted to use his brother’s room (he moved out last year) as an office.

I let my nephew know that the room isn’t free and if he wants another space, he’ll have to pay what his brother was paying; $300.  

So $600 a month total, $300 for a new office space and $300 for his current bedroom. 

My wife says I’m being unreasonable, but if he’s trying to build a business using my property, shouldn’t he pay for it?

I know he’s family, but my wife says I keep trying to extort him, but I don’t see it that way.  

Am I in the wrong?