Regardless who raises them depending on when soun dies would determine how Kasumi and Nabiki would turn out. Akane would likely be completely different, as it is she can barely remember her mother, Kasumi has a few more memories as she is older and likely started to fall into the same inital place as, well, caretaker of the family. Nabiki might just be worse as she became cold to hide her feelings, by losing Soun too she just might crack and either get worse or better, possibly going in the other direction, depends on who takes care of her, Akane would mostly build herself around who ever takes care of them, but since her early memories are of training with her father she might hang on to that bit of her personality.
As to the arranged marrage, it really depends on who takes them in. Some might allow it, others might reject it. It would be a hard fic to write without Ranma as a stable force. If soun dies before Genma goes on the trip though then it might be postponed or canceled. This one idea could rewrite everything. No Shampoo no Ukyo, no Ryoga, maybe no Kunos. Like a small ripple everything can change. With the loss of the Dojo would Genma even want to continue the engagment? Would he arrange others? Would Ranma even be in the fic at all?
Would Akane attend the same school? They lost the Dojo, as such they would need to live elsewhere.
A change of guardian for the Tendo sisters at that point of time could change the entire story completely, even having Genma lost holds less impact.
As such I recommend either way of someone who would keep The sisters close to original story or something completely different, if Ranma does make an appearance it might be an idea to keep his role minor in that case.
That is why I recommend Tokimi, she would manipulate events to keep the sisters close enough to their canon selves within the reason of her character and has the power to back it up, conversly she might go the other way, more options either way. Anyone else would likely try to skate in the middle ground which to me would seem more like someone trying to replace Soun with a Soun clone that acts different, AKA a Gary Sue. Who is chosen in the end won't matter as long as the choice has obvious changes to the characters.
Personally I would find a perverted Kasumi trying to grab Ranma for herself while fighting off other rivals to be entirely funny compared to Akane's semi-passive self (AKA she deals with Ranma as a problem rather then Ukyo or Shampoo most of the time).
As to the arranged marrage, it really depends on who takes them in. Some might allow it, others might reject it. It would be a hard fic to write without Ranma as a stable force. If soun dies before Genma goes on the trip though then it might be postponed or canceled. This one idea could rewrite everything. No Shampoo no Ukyo, no Ryoga, maybe no Kunos. Like a small ripple everything can change. With the loss of the Dojo would Genma even want to continue the engagment? Would he arrange others? Would Ranma even be in the fic at all?
Would Akane attend the same school? They lost the Dojo, as such they would need to live elsewhere.
A change of guardian for the Tendo sisters at that point of time could change the entire story completely, even having Genma lost holds less impact.
As such I recommend either way of someone who would keep The sisters close to original story or something completely different, if Ranma does make an appearance it might be an idea to keep his role minor in that case.
That is why I recommend Tokimi, she would manipulate events to keep the sisters close enough to their canon selves within the reason of her character and has the power to back it up, conversly she might go the other way, more options either way. Anyone else would likely try to skate in the middle ground which to me would seem more like someone trying to replace Soun with a Soun clone that acts different, AKA a Gary Sue. Who is chosen in the end won't matter as long as the choice has obvious changes to the characters.
Personally I would find a perverted Kasumi trying to grab Ranma for herself while fighting off other rivals to be entirely funny compared to Akane's semi-passive self (AKA she deals with Ranma as a problem rather then Ukyo or Shampoo most of the time).