The prophet warned against men entering upon ladies without permission or her mahram present. If your family is joined there may be sin comminted if you brother is at your home without you present or enters without permission.
Uqba bin 'Amir [RadhiAllahu a’nhu] narrated that the Prophet (prayers and peace be upon him) said,"Beware of entering upon the ladies."
A man from the Ansaar said, "Allah's Apostle! What about Al-Hamuw(brother in-law)?” The Prophet replied, “The Hamuw is death.”
this hadeeth clearly prohibits the brother in law being an exeption to the law.
In islam a women also has certain right, within those rights, a women must have her own home, unless she states that she is wiling to sacrafice that right. So if you want to live in a joined house, you must recive her consent. The reason for this is because a husband is supposed to be protecting his wife , and protection includes covering her from the eyes of non-mahrams such as a brother.
“… and live with them honourably…” [al-Nisaa’ 4:19].
Part of that means providing them with accommodation, because she cannot do without proper accommodation to conceal her from people’s eyes and so that she may go about her business, relax and her keep her belongings in order.
the prophet saaws gave each of his wives a seperate home, however he did not have any siblings or parents so i cannot give you a ruling on sunnah. Ali and fatima lived sepratly from abu bakr, this is the only example i can provide you on the matter of them living sepretaly from their parents.
in conclusion, it is possible for you to live as a joint family, however this would cause your wife to have to give consent of wavering her rights, possible risk of haram on your brother or wife, and maybe even discomfort on your wife. It may just be easier to allow your wife her own living space to avoind the possibilty of haram and discomfort.