hi,
u got to go thru a procedure for changing the name. even if ur sis is married, u stil ve to get it registered. for that, the processes is:
1. go to ur local MRO office n take the application from them for name n address change- they ll ask for police verification certificate n marriage proof n some cases a notarary signature on a bond paper(for this u need to go to ur local high court or district court n meet the notorary authority)
2. for police verification cert u need to go ur local police station n apply for it. they ll charge some 200bucks for it. one constable is sent to ur home for verification n then a cert is issued taht ur sis is indeed married to so n so person n is currently living in so n so address
3.u need to submit that cert in the MRO ofifce along with marriage proof, educational certs,notorary bond, ifany includig ur bro-in-laws edu certs.
4. then the magistrate signs on ur application n a reciept/acknowledgement/consent notice/permisson letter is given to u.
5. u need to submit tat reciept in the govt press for gazetted notification in the local newspaper.
6.once that is done, a name change certificate is issued to u by GOI.
7. then u can write a normal letter to the bank attachig a copy of this certificate.
unless u ve that certificate, ur sister's changed name is not legally accepted.