This type of queries require lots of information, after getting that, may someone gives you right answer.
Such question require more info about, family background of you & your bhabhi, age of bhabi, sexual life of your brother & bhabhi, no of yrs they got married, your mentality, your wish, your education, your sexual life, habits, family atmosphere, . etc
So I think, you do not expect exact answer but may you will get outline.
Such question totally depend upon person who involved in it, I am sure, everyone advise you that this is wrong but only involved person know what is limitation of her/his & situation of such incidence when arise?
But one thing for sure, be careful.