Selling Mom's House

My mom has been in nursing home since 2015.  We started renting her house in 2016 and it is still rented today.  We have received a very interesting cash offer from someone who wants to buy the property, but I am worried what the tax implications would be for this sale at this time.  I don't think it would qualify for the primary residence exemption since it has been rented during the last 5 years.

 

if she is required to pay capital gains - is it a graduated scale ie:  you clear $50k gain on the property...  the first $40k taxed at 0%, next $10k at 15%, and so on if she were to make more gain off the property?

 

Can anyone provide some guidance with regards to what her tax implication might beif we take this offer?  Otherwise, I will just continue to  rent the property until it would be transferred as part of her estate.