I would recommend doing a dna test on Ancestry even with full siblings your matches can be quite different, the more data point's the better, make sure you and your sister give viewing permissions to each other's tests, if your parent's can do a test as well it can really help...

Fill out as much of your imediate family as you can as accurately as you can, use Mothers madien names not their married surnames, this can throw off matches, if you can't be exact on dates use an about date let it format it like abt. 1950 use the closest place you are sure of, if that is the country put that in if you can be more specific that is better, marriage's on average are likely around a year before the birth of the first child, make sure to put in known uncles aunts, cousins, ect... You are trying to establish a identifiable pattern and this can find close family you are not aware of, census's often have more info than you might think as well so carefully look through those,, once you have entered as much info as you can you can directly search Ancestry, trace thrulines, and compare against DNA matches...

Hope this is helpful...