understanding some grammar
 

I am attempting to take my (very) basic Japanese skills to the next level. I have found a copy of the book 「チビ竜と魔法の実」and I thought that reading/translating it would help me. The first real sentence in the story is confusing to me. I would like to understand the grammar. Any help is appreciated.

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

It's mostly the middle phrase. Here's what I think I understand so far:

ユイにとって
Yui-to take (取る)
"For Yui,"

それほど大きな問題ではなかった
that-about big-problem-being was-not
"but that was not a big problem"

I think that I'm mostly not understanding the function of the particles in the middle phrase: の, が, だ

Taking the middle phrase part by part:
じぶんのママ
"my own mother"

が
not sure, but I'm leaning towards "and"

ほんとう
"truth"

は
subject marker

キシネ
proper name "Kishine"

だ
not sure

という
I'm not sure if I even broke this apart in the right words

こと
thing

ah, だ is the copula. :-P

>「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.