Their children (now in their late 40s) inherited the house, but they have not listed it on the market, and I heard that neither is willing to move into it from their current setup, so the property has been sitting vacant.

Quote from @Tim Buckingham: @Jonathan Moody Good information but I'd like to add some additional points (note that your mileage will vary due to numerous variables). 

Since making a career switch to real estate, I've been involved in numerous studies adding my expertise in statistics and housing policy with NAR, HUD, Fannie Mae, Freddie Mac.

You have 3 children and one just left for college.

I've analyzed numerous 1-4 unit residential deals for both cash flow and appreciation and haven't seen anything close to those numbers.  

Married with 2 children. 

No children or wife.

Optimally you also can get the children of the senior involved so nobody feels their parents were taken advantage of.  

@Caryn Fischer  The downside of gifting or selling property to your children is that it can have capital gains tax consequences.

@Eric SmithGenerally, if the taxpayer sells the relinquished property to an unrelated party, the taxpayer generally cannot acquire replacement property from a related party unless:The related party is also participating in a 1031 exchange.The related party pays more in tax on the sale to the taxpayer than the taxpayer is deferring in the exchange (this scenario is rare).Let’s look who is considered a related party: Spouse, children, grandchildren, parents, and siblings.Corporations and shareholders owning more than 50%.Commonly controlled corporations.Partnerships and partners with more than 50% interest.Trustees, grantors, and trust beneficiaries.Non-Related Parties:In-laws.Aunts, uncles, nephews, nieces.Friends.Domestic partners.Entities owned 50% or less by the taxpayer or a related party.In your case, your mother-in-law, aunts, and cousins are not considered related parties to you under the definitions in Sections 267(b) and 1031(f).