the second marriage will be void-ab-initio.
even if first wife gives consent (even written consent), 2 nd marriage will not be valid.
the man can be booked for bigamy.
but remember - either 1st / 2 nd wife can file complaint of bigamy - no 1 else.
if both the wives do not have objection, the man can not be booked for bigamy.
its the case like - Miya biwi raji, to kya karega Kazi !!
however, u can stop the second marriage, as it is illegal
sec. 5 (i), read with sec. 11 & sec. 17 - HMA, 1995
as per central civil services conduct rules, your sister will loose her govt. job if she performs this marriage.