My bet is that Ymir chose Mikasa because everything started with love and had to end with love. Meaning Ymir counted on Mikasa to be able to do what she couldn't, break the whole cycle by killing the man she loved. Ymir chose to believe Mikasa was, in the end, stronger than her. And she was right.
she saw the pure love mikasa had towards eren. thats why she lifted the curse. mikasa showed ymir the thing she wanted to see all her life, pure true love.
I think the founder ymir chose mikasa bc of the event of chapter 138, mikasa was able to say goodbye to eren in the most tragic way yet showing her love for him (not exactly the kiss, but u can see her smiling at the background of the panel) her attitude was a growth for the character, a detachment with a lot of courage that was something that ymir didn’t have in her whole life, so I believe it was motivating. And eren talked to the armin at the boat scene in chapter 131
I still don’t understand WHY the founder ymir chose mikasa? and also when did eren have these convos with armin and mikasa ? (=・ω・=)