Disclaimer: I do have personal experience with a people-pleaser, but not as a parent.
Christine Carter writes in Why It Doesn’t Pay to be a People-Pleaser (Greater Good Magazine, 2016):
I’ve spent the better part of my life as a people-pleaser, trying to meet other people’s expectations, trying to keep everyone happy and liking me. But when we are trying to please others, we are usually out of sync with our own wants and needs. It’s not that it’s bad to be thinking of others. It’s that pleasing others is not the same as helping others.
This is one of several arguments that may help you teach her, why people-pleasing is not good:
- Helping doesn't equal pleasing. She can very well help others without pleasing them (maybe even infuriate them instead), while she can please them to their own detriment.
- Such friends are not her real friends
- It's too one-sided a relationship and she needs to also take care of her own needs (and it seems that her sister is also suffering from it)
This may help her understand why it's not good. Actually, she may already know (some of) it. That's why you need to dig a little deeper. Her people-pleasing, although it may seem as if she didn't benefit from it, is a way to fulfill some need. You need to find out what it is, what the reason for her people-pleasing is, so you can treat the disease, not just the symptom. She doesn't just need to understand why it's not good, she needs a way out and the necessary tools.
You may observe her and her interactions with others, and her sister could possibly also share experiences. Have a conversation with her in which you mostly listen and let her tell you about why she does it. That means she needs an environment in which she can open up and trust you.
One reason why she is a people-pleaser may be low self-confidence. Perhaps, she feels that she can only find friends if she "buys" them and not in any other way. Then, you'd need to work on her confidence.
Perhaps, she looks for friends in the wrong circles, in which case you should help her meet other people and find new (actual) friends. You wrote that they don't do what she wants, so their interests do not align. This is a good way to search for new friends for your daughter - children with the same interests.
If she actually believes that she helps these "friends" and wants to help people, find ways for her to actually help people. Maybe she can volunteer - look for opportunities for children of her age.
In short, find out why she is a people-pleaser first. Then help her find healthy ways to find and keep friends.