[RadioKid]

>「ユイにとって、じぶんのママがほんとうはキシネだ� � いうことは、それほど大きな問題ではなかった。」

It is not such big matter for Yui that the her true mother was Kishine.

Though not all people agree with me, "は:wa" is not subject marker in this case. Rather, "が：ga" is the subject marker for "her mother".

"は:wa" is a marker for the main theme of the sentence. In this case, "ほんとうは" (meaning "as the true story") implies she has another "fake mother" than Kishine.