I'm going to be an Aunt :)

My sister is pregnant, and she's going to give birth to two beautiful girls.
Finally she's going to be a mother and I'm going be an aunt :D.

My sister's daughters will be the 1st grandchildren for  my parents. They are glad because they are going to be grandarents.

We can't wait!

Well done, I made some changes though. Gonna is not a real word :) Also, I restructured the sentence about the first babies in your family. 

I'm gonna be an Aunt :)

My sister is pregnant, she's going to give birth to two beautiful girls.
finally she's gonna be a mother and I'm gonna be an aunt :D.

My sister's daughters are the 1st babies we have had in my family, my parents are glad because they are going to be gran-parents.
we can't wait for that.

well writen, and congraduations