After you have researched Tenant’s Rights in your locale, give as much notice and help moving as you can afford, especially if she has been a good tenant.
Years ago we rented a home, and upfront knew the landlord had purchased the house to eventually be a home for his elderly parents. When he told us a couple of years later that it was close to time for his parents to move in, we were both very civil about the time it took for us to find a new home, and when he needed it. Friendly conversation was the key.
Actually you SHOULD feel bad about telling someone they have to move. Although you purchased the house for your own use, and it’s your right to do with it as you wish, you are forcing them to relocate. And for most folks moving is a real inconvenience.
You don’t have to feel guilty about kicking them out of their home, but you should treat them as you would like to be treated if you were them.
You absolutely SHOULD NOT feel bad about telling them that they will need to move. They are RENTING/LEASING their home; being told it was time to move on was (or should have been) always a possibility.
What you can do is be respectful, and give them as much notice as possible… "Hi Mr/Ms Tenant, you have been great! However, I am going to want to move in before Christmas. Let me know when you’ve found another place. I can be a little flexible, but I really want to move in by then. "
Sure it’s hard, but first is to check what her Lease says. Follow the lease! Some states have some really ugly Landlord/tenant laws, so as someone else said, Know the rules! You could get into a real fix, especially if you are in CA or even FL. Everyone is correct about talking to them, giving plenty of time, and being sympathetic to their situation, but follow the lease and the law first. Do it all in writing and do not hesitate to use registered mail. Then you can be a caring soul and sympathetic. They could wind up owning your house in some places. They may be sweet but may have angry relatives.