I was interested to marry one girl. While after I got to know that she was my maternal uncle's sister. But he is not my own mother's brother.
Can I marry her or not?
It's difficult to answer given we don't know quite what you mean by 'maternal uncle sister'; we don't after all know which social-cultural tradition you're from.
In the tradition I know, Bengal - maternal uncle would mean
an actual brother of your mother
he is actually related to your mother as a brother, he might be a cousain say
he is a close male friend of your mother; we then also say 'maternal uncle sister' for his daughter, which we regard as a sister.
Which one of these is it? It's obviously not the first; so it's between the last two; on what you say, it looks like you can marry her - but it's good to be clear.
You can marry her, there is no restriction from Islamic view.
Don't forget to make Istikhara before marriage, so that you could get help from the Almighty.
May Allah accept your conjugal life.
All praises and thanks are due to Allah. And regarding the answer of your question , marrying is one of the legal ways of our Prophet Muhammad (p.b.u.h) Therefore, you can marry this girl and there is nothing wrong with that. And the Almighty Allah knows best.
و جزاك اللّٰه خير الجزاء والسلام عليكم ورحمة اللّٰه وبركاته