Idiopathic aplastic anemia 21y

So my sister is having anemia for past 1 year. Only Hb and RBC counts go down every month with low retic count on bmbx twice .25 and .1 in the 2nd one, so it's decreasing.
Workup done till now:
Pnh, NGS cytopenia complete panel, ANA by flaer, Dct ict also by z method, parvovirus b19.

Normal healthy individual with enough weight and everything 58kg. No other symptoms other than only when the blood count is low then weakness, heart racing, urge to sleep etc the classic anemia symptoms...

Only rbc/hb is affected, rest WBC and platelets are ALWAYS within normal range.
What could be the problem? Has anyone here faced a similar issue?