Depending on when these properties were purchased, along with a handful of other factors, these two statements could be mutually exclusive.  

Yes, you'll owe tax on the gain. you're well outside the two years residency out of the last five for the 121 exclusion.

Where i have seen owners get fubared is they have a renters policy and there is an exclusion for when the house goes vacant and they do not know that and then something happens to their rental and they get denied..

Give them a call and just ask them if the information is published in the newspaper.

Your brother-in-law’s 10% ownership won’t affect your ability to claim this exclusion on your portion.Unfortunately, the option to roll proceeds into another property (like a 1031 exchange) only applies to investment properties, not personal residences.You’re likely eligible for the exclusion, but I recommend confirming with a Tennessee tax professional for any specific state tax issues and to verify your status with overseas residency.Let me know if you need more guidance on this or related financing!

Just because you come to terms on price you do have to also do due diligence before you sign the contract and become exclusive.  

Capital gain can be avoided by using sec 121 home exclusion (upto certain limit).

For example we write exclusive use of the rental property includes use on two sides to the tree line, on one side to the rock wall and to the front curb. 

From a sales perspective, if you're married ($500k) or if single ($250k), you'd have a portion of the exclusion (%50) assuming the square footage is the same for each unit (assuming a duplex)The remainder (%50) could be 1031 exchanged into another property tax deferred. 

The past five years here in Oregon I've focused almost exclusively on investor related acquisitions and sales of vacation rentals, multi family complexes and commercial real estate.