Agreeing with the advice you've been given so far. This person is a stranger. Becoming a triad would be like getting engaged to a mail order bride. You don't know this person from Adam. Sure any opportunity for new love - be it brotherly, romantic, or other - is always great, but jumping into a relationship shouldn't be something you feel pressured to do...even if the pressure is only from theorizing that "it would be better if we started the relationship this way".
Just go, be breezy, discuss your personal limits without being oppressive, and all will work out fine
Me - male, 42, poly, straight, in a serious relationship with Audrey, also casually dating.
Audrey - female, 20, poly, pansexual, also casually dating.