I hate to ask but…

Grammatically, you've split the first part of your sentence "After discovering they're adopted..." from the rest of the sentence. So replace that period with a comma, first up.

You've described TWO protagonists, when it helps to identify who the central protagonist is - whose journey we're going to follow. Even Thelma and Louise or The Shawshank Redemption have protagonists.

Taking a road trip to find a compatible bone marrow donor is not a logical action to take after finding out you're adopted; my guess is that, while finding out they're adopted might occur in the first act, this is not actually the event that kicks off this story.

So - refocus the POV to a central flawed protagonist, adjust what the event is that kicks off the story:

'When only a bone marrow transplant will keep her alive, a defensive adoptee crosses the country with her burnout sister to find the only compatible donor: her biological mother."

I'd suggest trying to find the source of antagonism; who exactly is making this road-trip difficult for the characters?
Likewise, it'd be very easy to include a time-clock to up the tension.

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

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