They were on their way to visit their brother, a [something] in the Constantinople [something]. As they approach, however, they see the siege in progress, and ... things go awry from there. Their paltry, provincial protection charms, though well-suited to the standard hazards of travel, are quite insufficient to even get them past the siegeworks, much less do anything about the great and terrifying goings-on. (Their aunt -- who cared for them since their mother passed away when Velma was three -- of course equipped them with the standard travel protection charms, but had she known to expect such a grand and frightening thing she'd never have let them go in the first place.)
They try a few things to the end of entering the city and doing something, getting their brother out, or giving him aide, or anything that they can -- but without success. Perhaps they enter the city and escape with him; perhaps they encounter him as they make yet another attempt and he's already in process of leading refugees or something away. That one sounds better; then we can have a sort of road story/quest/journey. Their detour takes them not only in a very unexpected geographical direction, but also in a very different ... vocational ... one.
I need to read up on the siege of Constantinople.
I basically want their brother, whatever his name turns out to be, to be a great, tragically doomed, heroic young man they all love very much. He should do great things. He should die in some final dramatic something, perhaps staving off some imminent disaster (hm?) while the sisters undergo whatever existential transformations are involved in becoming -- all of a sudden -- the three Fates.
His loss is absolutely devastating.
The sisters, perhaps by accident, are wrested from their former selves and suddenly must cope with not only immortality -- well, sort of -- but also the sudden onslaught of an unfathomable amount of brutal, pitiless knowledge. The former Fates meet a simultaneous and quite gruesome end, perhaps involving something to do with the and the sisters are the only acceptable candidates nearby, and they are thrust into this epic responsibility without any of the kind of preparation and mentorship that the position normally involves. They have to cope with all this thunderous new purpose and burden in addition to dealing with their human, fraternal, immediate grief. They three must create the role of the Fates anew, because the only ones who could have guided them are gone. They three are all they have.
Perhaps the reason there have to be three of them is because if that burden fell completely on just one, they'd be crushed beneath its insupportable weight. So they share -- and struggle over -- it. Like a sickness one kid brings home and then the rest of the family passes it around from person to person. Hm.