I think what set off alarm bells is that he is trying to fix you up with someone he likes. I have found that trying to force a triad may lead to instability. Triads are hard to maintain because everyone has to change in the same direction in life. Or else one relationship weakens to where it would want to be a V (vee), yet people try to keep it a triad.
If you do pursue this, then approach the relationship with her separately and let it evolve into what feels comfortable. Maybe a friend, maybe a lover, maybe just an acquaintance.
As for jealousy, one thing I have found is it is good to recognize it as a legitamate emotion which may tell you something deeper about yourself. It may be envy, insecurity or a bad association. For example, maybe you are worried he likes her best for one of her attributes. Understand it is insecurity and then talk to him about it. he can try to reassure you by telling what attributes about you that he likes.
I wish you well.