Raising Rent on Long-Term Inherited Tenant?

[Jesse Smith]

I am considering offering on a duplex on the same block as one I currently own. The two properties are nearly identical... 2/1 on one side and 1/1 on the other.

My units rent for $1,100 and $900, respectively.  The rents at the one I'm considering are currently $700 and $550, respectively.  The owners are long-time, absentee landlords, who inherited the property.

I learned this, because I met the tenants recently.  They stopped by while I was rehabbing my duplex last month.  Both are very sweet ladies who have lived there for 8+ years each.  They were shocked at the rent I planned to charge and volunteered what they paid.  They proudly stated that their rent had never been raised.  They even gave me the owner's cell phone number when I asked who they had used to install the siding.

So, now I am considering calling the owner to make an offer.  If I do, I know what market rent is, and I would want to get to that amount.  This would represent a substantial increase in rent for these ladies, and I feel a bit bad about it.  

What would you do in this scenario?  How would you approach raising the rent to market rates?