Center my dedication in my thesis

use star with \vspace:

\vspace*{4cm}

Have you tried this? BTW I would write my sister first, then the brother.

\documentclass[a4paper,     % Seitenformat
        12pt,                               % 
        bibliography=totoc,     % Literaturverzeichnis in das Inhaltsverzeichnis
        index=totoc,    % Index in das Inhaltsverzeichnis
        abstracton,     % mit Abstrakt
        headsepline,    % Trennlinie f�r die Kopfzeile
        %footnosepline, % Trennlinie f�r die Fusszeile
        ]{scrreprt}


\usepackage{calligra} % for dedication

\begin{document}    
\title{Title}
\author{Joe Doe}
\dedication{To my parents, my sister and my brother.}

\maketitle

\end{document}