Your brother does not need to understand fully or even to approve for you to have a relationship with him. If he can't accept that a psychiatrist diagnosed you bipolar and you are on medication for it, then that's his problem, not yours. My guess is the idea scares him, especially if your mother was bipolar as well. I'd maintain a kindly relationship with him, and if he asks, just tell him what the psych said. You can't "prove" you have an illness to anyone, and it's rude of them to ask you to.
Of course you should have people around you who have your best interests at heart, but I question that hooking a married woman up with an old friend for an extramarital relationship is an example of having your best interest at heart. I'm sure he was working because you asked him to, but he should have said no.
I'm glad in the end you discovered your husband is your true love. Believe me you're not the only bipolar to go through these fickle emotional questions. (Read any of the bipolar spouses' stories on this board.) But it's good that you're now working to have a healthy and stable relationship with your husband. Staying on your meds will help prevent this from happening again.
Co-Moderator, Bipolar Forum