She has been a Christian since age 16 and has served the Lord all her life. Like most Christians who grow up and stay in church she has served and ministered in a variety of capacities in the life of several churches.

I went through a very similar process at one time, but I was in my early 30's at the time. That's why what's happening in her life right now seems so remarkable to me. Age 70 is a time when most people want to sit back and reflect on what happened over their lifetime. It's not generally when people are saying, "God, what is this new thing you are calling me to? How can I use my life to serve you best?"
During the years when Louise and I were traveling across Canada doing children's ministry, I frequently had a chance to speak to the seniors groups in the churches where we were ministering. I taught a session I called "Seniors In Service". The main point of this seminar was that God never puts us on the shelf, and that regardless of our age He will always have some way we can serve Him. We let the concept of retirement creep into our service to God, and that's something God ever intends for us. While the kinds of things we do change as we grow older, i.e. the nature of our service changes, God will always have ways for us to minister. Retirement is not a Biblical concept.
These things were easy to say to senior people when I was in my 40's. Even then, as I was challenging seniors that way, I just knew those words would come back to haunt me one day. Now that I am in my 60's, and not experiencing the same kind of energy I once did, I can see I am going to have to constantly challenge myself with the very message I was teaching people as much as 20 years ago.

The only person God is ever finished with is the one who refuses to serve any longer. For those who are willing, God will always open doors of ministry. He will always provide ways in which we can be useful in ministry to Him and to others. We can always have an impact regardless of our age.
So my sister, in her retirement years, is clearly sensing God doing a new thing in her life and she is anxious to discover what God has in store for her. In His timing He will reveal that to her, and I believe she has some exciting times ahead, ministering as God leads her.
Are you open to what God wants to do in your life, regardless of your age? If God says, "I have something new for you" are you open and eager to serve however He leads? The rewards of serving God are amazing at any age. Don't rob yourself of experiencing Him working in and through you.