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}