英语翻译，各位帮我看看

I think the most significant thing of the Spring Festival is the dinner on New Year's Eve.

significance词性错了，最重要要加最高级。有了I think,for me 就多余了。

要么说younger cousin, elder cousin,要么就说younger sister, elder sister

The most important thing for me during the Spring Festival is the New Year's Eve dinner.

如果说表姐，只要说my cousin Susan就可以了，不用说big sister 或者 younger sister 叫姐或妹是中国人的习惯，英语中只说sister或cousin就可以了
my sister Lily, my cousin Susan

It is the family reunion dinner on New Year's Eve, from what I believe, that the most important part of the Spring Festival.

my cousin xxx, and cousin xxx.

glad I can help^^

I think the most significant things during the Spring Festival is the New Year's Eve dinner.

And, in english, it is not necessary to indicate that who is your elder sister or younger sister.

you use "my sister" is good enough.