Can speak as a runner but not a spectator.
You may have your (very good) reasons to want to see them in the MK/on Main Street and you’ve gotten the advice on what to do… runD bus to Epcot that AM (for the Marathon those buses run 2am-4am), Epcot monorail to TTC/switch to Resort Line to MK (starts a 4am), MK spectator viewing is 5am-7:30am and is ~Mile 9.
I’m sure you’ve seen all the various start time/transportation time/spectator info here, but just in case: https://www.rundisneyexpo.run/walt-disney-world-marathon?
A lot can depend on their start time and speed, but at Mile 9 the course is still often congested and typically there are A LOT of spectators. With the
amount of spectators, runners, noise, music, and chaos it can be hard to see people (and for them to see you). But of course it is Main Street, it’s awesome, it’s a unique way to be in MK, etc.
As a runner, for some that can be a rather emotional moment in the morning - the culmination of months of training, days of early wake up calls, you run through those gates, step foot on Main Street, make the turn by the Confectionary and see the castle in the distance - it’s all the feels, for me at least.
So yes, it can be done, but I do tend to agree with others I wouldn’t rule out trying to see them by BWI/BC/or Epcot towards the end of the race. It would probably be easier, on the margins, but would obviously be much different timing and may impact other plans. You could also plan to see them both places!
Maybe obvious statement, but pay attention to the weather - eg, last year it was COLD on the Marathon morning and those spectating very early really needed to bundle up. Typically even when it’s cold once the sun comes up and the day starts temps are more tolerable as the morning goes on.
Sorry for the unsolicited advice there. Good luck to your sister and her daughter, and enjoy spectatating! Such a special event/day for everyone involved.