Excuse me man! Are you asking us for permission or are you asking us a suggestion??
Kind of your question seemed like the former..
Anyways to your question.. I or anybody in this site who has an ethical and moral sense would never recommend you incest.. may it be close or far relative circle..
Speaking genetically, men are programmed to get attracted to women with the looks and body.. It happens with even with the closest of the family members.. But then.. It the reason why this is forbidden is that it prevents variations in the genes if the gene of the male and female are retained within the same gene pool.. Also there are chances of getting more genetic diseases for the offspring if done in the family..
Now to your question in more detail.. You may use a condom and have sex claiming all the above claimed bull is crap.. But how would your bro feel if he knows his younger bro is sleeping with his wife.. Or let me ask you like this.. How would you feel if ur wife or girlfriend if sleeping with your elder bro??
So dont do anything.. And if u say that ur bhabi is a ****, warn your bro and ask him to so something severe about it.. coz its his future and your family's prestige involved..