So I started to look into online features, to see if it had some sort of session requirement like the sunbro covenant. I have done a lot of low level co-op sessions in blighttown lately, but that didn't help either. So I started to check if she was there during co-op sessions, and she was there more or less every session the host had summoned a powerful player.
So I tried it myself, and after summoning my first sunbro she appeared. So it looks like a +10 glove is required, but you don't need one yourself, any of your phantoms can have it.
I also tried to test if she appeared when being invaded by a twink (with a +10 glove) before I tried to summon a phantom, since I only had one shot at making her appear, but I never got invaded.
There might be more to this as well. Have any of you made her appear without a +10 glove, either offline or with no summoned phantoms?