Is it permissible for the eldest sister to be the guardian of her younger sisters and their wealth after the death of their parents, especially since their Muslim relatives reside in Islamic countries?

The sister is not to dispose of her sisters' money unless the father has willed it. If the father has not willed it, then the matter must be referred to the Sharia (definition) Court or the Islamic Center to appoint a guardian for them. The sister can be appointed as their guardian, as the majority of scholars have permitted the guardianship of a woman, citing that Umar (may Allah be pleased with him) willed to Hafsa, and because a woman is eligible to bear witness.