If she's stronger than Lanji then she will not join, very simple.
Jinbei is fighting a F6 while Lanji is fighting a YC 2. Also, you can say Jinbei mid diff Who's Who because the fight is not over yet.
Whoever joins will never surpass the monster trio. They will always be the top three in the crew.
Do I have to remind you of how Franky was portrayed before he joined the crew? He equally fought against Luffy. Or Robin in Whiskey Peak + Alabasta? If Yamato joins then she will be weaker than Lanji and Jinbei.