Complex sentence: While Kuroka did not mind him using his servants as test subjects (or herself), using his servants' family (namely Koneko) was beyond the pale, especially since Koneko was extremely young at the time and her powers were still unstable.
Split into simpler sentences: While Kuroka didn't mind him using his servants as test subjects (or herself,) using his servants' family (namely Koneko) was beyond the pale. Especially since Koneko was extremely young at the time and her powers were still unstable.