Complex sentence: Proto-Slavic and its development into today's Slavic languages have been reconstructed using the comparative method (which has also been used to reconstruct its mother tongue, Proto-Indo-European).
Split into simpler sentences: Proto-Slavic and its development into today's Slavic languages have been reconstructed using the comparative method. The comparative method has been used to reconstruct Proto-Slavic (and its mother tongue, Proto-Indo-European).