My sister in law passed away and left us her house. The house still has a mortgage which is in the s.i.laws and her ex husbands name(he signed a quitclaim deed long before she died)

We would like to finance the house into our name but don't really know where to start.

Whoever is the executor of her estate can sign off , 'selling you' the house, (which pays off her mortgage co with $$ from your financing) or you may be able to take over her mortgage.

If the ex signed a quit claim, then why is the house still in his name?

But lauren is correct, the executor of the estate has to deed the property from the estate to you at which point you can get your own financing.