Alucard was not actually with the siblings as a canon relationship in the main Castlevania material; the most relevant source here says Alucard and Harith are not related , and Harith became more of an ally/adopted younger brother after Alucard saved him. If you meant a different ā€œsiblingsā€ pair from a forum thread or another adaptation, the wording is too ambiguous to pin down a duration from the available source.

What the source suggests

  • Alucard and Harith are described as coming from different families, not as blood siblings.
  • The relationship is framed as ally/adopted-brother rather than a timed ā€œwith the siblingsā€ situation.
  • A Reddit discussion also notes that a similar pair people ask about were ā€œnot siblings,ā€ which points to common fan confusion.

Simple takeaway

There isnā€™t a verified length of time for ā€œAlucard with the siblingsā€ in the source I found, because that specific sibling claim doesnā€™t hold up in the material returned here.