Both
 

Both sisters mouths are the same. We can conclude that these two mouths were both drawn wrong (in the same way) or that this great portrait painter who was awarded honorary doctorates to Harvard and Oxford by age 40, painted 600 oil portraits, was offered a knighthood, goofed.

It's all too easy to paint careful, boring, safe portraits. Daring work is more interesting. Artists talk about it 110 years later.

I never noticed anything special about the mouth but often wondered why a hand that looks as thought it is about to receive a train ticket emerges from around the waist of the daughter on our left. The painting would not suffer without it. Daughter on left also pushes the dress of her sister and causes a shape that looks like a deformed leg under the white dress. The over all effectiveness of the painting is so strong that these things don't much matter.

Sister Equality
 

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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Illustrator Harry Furniss was apparently intrigued by that hand, as well, exaggerating the gesture in caricature, in his

Now that I see the cartoon, the sister in the back is rather "beefy" with very wide shoulders. Someone I'd hate to arm wrestle... Great cartoon!

Thanks for the cartoon and link. I've been reading about the Sargent Murals all morning.

Note: before my post, there was a post talking about if they were not married, the pose could be flirting.

Were they married? A good point. Another point is "Jewish girls are more active than our English girls" (not exact words)
None of E.D.Boit's daughters were married. The girl posed in profile lived twenty some years less than the others, as if doomed by her obscure appearance in the picture.

Hummm...if you mean the Boit daughters they were married. They bequeathed the portrait to the BFAM in their old, married age.

Mouth, schmouth.

The KILLER for me in this painting has always been the way Mr. S. painted the fan edge-on. It makes the line of the arm so much longer and more sinuous (and sexy!).

There is a secret in the design of "Daughters of Boit", if you already know the answer, please postpone your answer, so let other people to explore.

Huh?
 

SBW, I didn't get that last remark at all.