Hi,
yes you can file divorce on mutual consent as your sister married for more than one year. Yes it is necessary that a mandatory period of one year separation is needed but if both parties are agreeing on the date of separation now they can go ahead and file a mutual consent divorce
The petition has to be signed by both parties and filed jointly by the husband and wife . The disputes related to alimony or maintenance or shtreedhan (wife's gold, articles , gifts etc. )to be returned etc. have to be settled between the parties before the filing of the divorce on mutual consent.,his can be done by an advocate who can call both parties and prepare a consent Terms and file it in the court.
The Mutual consent Divorce will take six months time tfor the final order in normal case, and in case of any disputes not settled as per the consent terms of the parties, you can keep the petition pending up to 18 months befor agreeing for the final consent and submission.
If both the parties are agreeable you both can approach one advocate and save money in terms of advocate fee.