In my house, we just add new systems. You won't make anything selling a used Wii - maybe 20 dollars! And the accessories - go ahead and check what Amazon offers as a trade in. You'll be shocked - it's not worth it.
But then again, all that is beside the point - what other responders have said is absolutely right. You have ANOTHER child that uses that toy. How could you even think of selling it? "Sorry, child one, I'm selling the family toy so your brother can have something that has ZERO games for girls."
Personally, we stick with Nintendo because it's more family friendly. I've been with Nintendo since it's introduction of the NES, which I STILL have. I don't think the Wii is for babies, contrary to apparent popular belief (I am cringing, honestly - my husband plays plenty of games on our Wii. Shop online for your games - there are plenty!)
The Minecraft is a fad. Do you want to spend the money on a fad? Go ahead - totally your choice. If it were me, I'd have your son help to earn the money - perhaps you can match him as part of his gift - to see if he really is invested in doing it. It's too easy for him to sell his sister toy to buy his own, and frankly, a bit ridiculous. If he has to earn a part of it, you'll truly know whether he really, really wants it - selling the old Wii isnt earning it. ;)
For Christmas, I got my oldest the 2DS. The DSiXL got handed down to the youngest, who had JUST figured out how to play and enjoy it. I would never have thought of trading up.