I've finished the section with the drawing instructor and begun the section with the lawyer. I thought it'd be fun to post my thoughts on the mystery so far and see how it measures up after I've finished the book.
Spoiler:
My first thought on meeting the family was that Collins makes sure to describe Miss Halcombe as manly and the uncle Mr. Fairlie as womanly. I thought perhaps this could point to someone somewhere in the story appearing to be someone of the opposite sex. Definitely if this were a more recently published book I'd think that. As it is, I'm not sure this book will take such a dramatic turn as that, but I'm holding the possibility open. My inclination at the moment is that Collins may have described them that way for two separate reasons- to make it obvious why the drawing instructor wouldn't fancy Miss Halcombe as any sort of romantic interest, and to give even more reason to dislike Mr. Fairlie. I do however still hold open the possibility of people posing as other people, cross-dressing or no, and I do note that Mr. Fairlie is rarely seen and when seen it's only in a darker room with no sunlight coming in.
My second thought was to suspect everyone of mischievousness in turn. Could Miss Halcombe be orchestrating some elaborate ruse that would result in her inheriting the money? Or could Mr. Fairlie (or whomever they may be) have a reason to orchestrate something? Or could they even be working together, or be the same person? If any of these hold true, it would mean they sought out a drawing instructor to come to set their plan in motion, making the instructor one of the victims/pawns. The plan could have called for finding an instructor that would be likely to fall for Miss Fairlie, setting up the scene as it were. This could even also implicate his Italian friend who found him the job, whom for all we know faked the drowning so as to have a reason to owe the instructor a favour.
Then there is Miss Fairlie herself. Could she somehow be in on it? Or is she another pawn in this that the drawing instructor or whomever will eventually need to help save? Or is she even Miss Fairlie? There is the possibility the Miss Fairlie at present is actually Anne Cathericke (I'm not sure I have the name right, but it's close enough). Miss Fairlie and the woman in white could either be the same person, or they could be separate people but switched (the real Miss Fairlie being the woman in white).
The points in favour of there being a ruse against Mr. Hartwright (I can't remember if it's spelled Hartwright or Hartright) in one way or another is that there are so many coincidences. He just so happens to meet a woman on the road to London that lo and behold ends up being central to a plot of the far away place he is about to go to. And after he drops her off, he just so happens to be at the correct spot to hear some men discuss looking for her. And there are more as the story progresses. However, these types of coincidences are more common and more accepted in older books, so I can't say for sure if they mean anything in particular.
If the whole thing is taken more innocently - that the woman in white, Miss Halcombe and Miss Fairlie at least are all as they say they are without ulterior motives - then I must look at things in a simpler way. There must be a reason for Miss Fairlie and the woman in white to look so similar. If they are not the same person, then I think maybe they are sisters. It is stated that the woman in white's father abandoned them and she doesn't know who he is, and it is also stated that Miss Fairlie looks like her father. So, if her father Mr. Fairlie had an affair and another child, and they both were born around the same time and both bear a strong resemblance to him, then we could have the situation at present.
I don't know enough about Sir Percival yet to make any particular guesses, but my first instinct is that he is going to be on the villainy side of things.