Because you have asked the question, I will answer very honestly. I think you sound really ungrateful. Here is this woman who has always taken an interest in your life and now takes an interest in your children. She gives freely of her money, but more importantly she gives freely of her time. You have posted about members of your immediate family having 'priority' over your great aunt, but I'm not sure that my priority system is the same as yours! The people I prioritise for my children to spend time with are those people who love and value my kids and who will put themselves out to spend time with us, not the people who happen to have been born in a certain order.

Do you need to set some boundaries? Absolutely! You are the only person who can determine what those boundaries are, but as someone who has very little extended family and who would LOVE someone to be chomping at the bit to babysit my kids... If you don't want her, I'll have her!