I think there is a potentially compelling story here.

What if the film opens with the ill sister getting the grim diagnosis.? And after she asks her family for a donation that matches, someone fesses up that she’s adopted.? Which triggers the need to go on the quest.? IOW: the diagnosis is the root of the action.? And the discovery she’s adopted is the cause? (trigger) of the action — the plot.

So maybe something like:
After a young woman in desperate need of a bone transplant discovers her family can’t donate because she was adopted, she must find a biological relative who can.
(28 words)

Another thing: It seems to me that one who needs the match is the more powerfully motivated of the two, hence, the story’s protagonist.? ?The other sister is in a supporting role as an ally — but not necessary to mention in the logline.

Further, why does the other sister have to also be adopted?? Why can’t she be a natural born member of the family who has bonded with her adopted sister and (unlike anyone else in the family) and, therefore,? is willing to help?? Contrasting characters on a road trip/quest make for more interesting dialogue and action moments than similar ones.

fwiw