Amber said - Nell is blind. So she is often with Larita who acts partially as a seeing eye. The two are probably the closest when it comes to sisters.

#330: And theres the rest of the family.

• Why doesn't the one with the peach hair seem as excited? She seems much happier later on.
  Perhaps she is not entirely comfortable in the surroundings because she is blind; or
  Perhaps she is keeping an eye on her blind sister
  
• Also, note the black one standing right with her - their arms almost seem linked (but probably aren't).
  

From this comic, it would appear that these two are Larita and Nell. Which is which though?

#338: They aren't scorned, but does it matter?

• The two I singled out as Larita and Nell are right next to each other
• The black one doesn't seem to be in a position that would be comfortable for a blind person in an unfamiliar environment. So I'd guess that this is Larita - although it's by no means solid (eg they may visit often).
• Note all their hairstyles and outfits - to me, the peach haired one has a relatively simple hairstyle and her dress is the odd one out amongst the sisters.
  Stereotypically, this would make her the most likely candidate to be blind.

#351: Before the hero...comes the tree-climber.

• "He does? I want to feel them!!" - why question the fact when Dan is in plain sight? Also touch is an important sense to blind people.
• Note the two are together yet again - the black sister kind of appears to be leading the other one (but that may be due to overcrowding of the frame)
• Again, the peach haired one is wearing a full dress - putting her at odds with her sisters (although Dan is obscuring the one in front of her)

Using typical comic design, characters who haven't appeared and/or won't appear in a scene never talk - it's just confusing for the reader... so Nell is probably one of those four.

PS: I deliberately chose not to overly criticise my points... it's hard to create an affirmative case when you've got barely anything to go on =p