What tenant needs to leave?

[Jerod Smith]

 I currently have three, one bedroom cottages on our property along with our personal home. All are rented at the moment, but my wife's grandmother needs to move into one because my wife is her primary care giver and she is elderly. 

 Tenant number one has been there 15 years, doesn't pay very much rent, but is a model tenant. 

 Tenant number two has been there for 12 years, been late on rent several times, has a dog inside the unit and smokes inside the unit.  Both of which are against her lease.  (This one is pretty outspoken and will give me major grief if I ask her to leave.  Her grandkids lived in unit 3 and left on bad terms)

 Tenant number three has been in the unit less than one year and pays the most rent, keeps her unit clean, and has never been late on rent. 

 All three are ladies in their mid 60s.  What is the correct philosophy concerning who should move out because of our family situation? 

Thanks in advance!

Jerod