there is 2 very strong schools of thought on these sorts of things. My personal view is that a dom/sub relationship between two humans, is going to result in emotional attachment to one another and actually makes everything stronger. The obvious problem here is that she is involved with someone else.

What it comes down to, is if she loves you two, she needs to commit to one thing or the other. That shouldn't be something you force upon her or pressure her into, but is the way it needs to go eventually (poly with 2 doms, never good!)

Regards,

Arch