The OVA will definitely not cover the SISTERs arc, that's clear enough from the PV. It makes more sense for the SISTERs arc to be the focus of season 2 of Railgun, and have the manga material take up the first 13 episodes of the season and have the second half of the season cover the aftermath of the SISTERs arc with filler written by Kamachi (which technically makes it non-filler), similar to the model they used in the first season.
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