x1 is path-connected to x2 via the female-only first-born progeny/primogeniture relation; x1 is the first-born daughter of the first-born daughter of ... the first-born daughter of x2, connected by a graph-path passing through intermediate relations/steps (successive first-born daughters) x3 (ce'o; ordered list).
Not necessarily restricted to only females iff the case can be made of female-filial/thygatric/daughterial relation despite gender; x2 need not be female. Will typically be restricted to biological or adoptive relations (otherwise, what is the point/what does the word mean?). Append with seltau "sarcu" for qualified/conditional primogenture; append with tertau "cerda" in order to reference primogeniturial inheritance.