how about getting a property with an annexe or extra rooms to accommodate him so that you can have the best of both worlds? i appreciate the comments above about how important it is for you to be happy and lead your own life, but how far is your lady willing to compromise and share you with your brother? she has, after all, pursued a relationship with you knowing all about your brother and has she given you an ultimatum (i.e. it's me or him)?
if the latter is the case, are you willing to make drastic changes (and maybe irreparable ones) to your and your brother's lives for someone who has done this? it may be a warning sign that she will continue to behave this way in the future with you - maybe saying and pressuring you into things you do not want and could cause you deep unhappiness and issues down the road.
i see no reason why you cannot all live together happily and lead productive lives all under the same roof - i currently live with my hubby of 20 years and his brother moved in when our son went to uni last year. he has chronic paranoid schizophrenia and is bloody hard work - but i would never give an ultimatum to mr kicker over this. it has caused lots of problems and we work them out together and discuss things in great detail to make them work.
next year, when our son finishes uni, if he decides to return home, we will have to up sticks and get a larger property to ensure everyone has space and is happy (enough), even though i will be devastated to lose my current home. and, considering the circumstances for us, my son will probably end up in the annexe as that would be the best fit for all concerned. but - i would do this for all my boys and myself!
there are ways through every issue and loving couples make huge sacrifices for each other in order for relationships to grow, change and adapt to all the events life chucks at you. discuss all this openly with your partner - if she is not even approachable or willing to consider this for you (and your brother).....how on earth are you going to survive being in this relationship? it's not something that i would tolerate in a life partner. i wish you all good luck x