we have aredhel, in some versions, galadriel, idril and celebrían (since only children are still youngest children), lúthien, elwing, arwen, the unnamed daughters of aragorn, morwen, rian, emeldir, nienor, haleth, ancalimë, eowyn, lothiriel - i could keep going across families, time periods, species.
this struck me really hard because in my cultural background, and sadly many others, parents often keep having children until they get a boy. especially in older generations it's more common to see a youngest/only child who is male because they are the goal, while girls are seen as less useful, not expected to work and inherit businesses or even move out, unless it is to live with her husband.
so yeah, probably just a cultural difference, and definitely not what jrrt was thinking about. but really unexpectedly heartwarming to see so many families ending with a girl.