Jim,
Maybe the hand is there as a way to bring the sister who is in the back in white onto the same plane as the sister in red. A visual placeholder to make them equal. Imagine if the greatest portrait artist today painted you and your brother (for example) and put you in the background? If it were me, I'd be ticked. Sibling rivalry goes deep...
In addition I think the hand adds a seductive quality to the portrait that it would otherwise not have if the hand were not there. As if the hand, in conjunction with those smiles, are asking the viewer to join them and share in their pleasant surroundings. Rather inviting to say the least... Were the sisters already married? If not, this may have been a way for them to meet wealthy suitors. Especially if the piece went to a show or exhibition for all to see. Who knows?
Gene
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