Monica And Ross
After I see the series I have a big question that Which one is older between Monica and Ross?
At First, I thougth Monica is an older sister but when I checked the lines in the show, I realize that I misunderstood.
At the lines below, I show that what make up my conclusion.
Ross – About Age
The pilot episode , "I’m only 26 and I’m divorced!" – Air 22 Sep 1994
In episode , "I am 30 years old, and I’m about to be divorced twice." – Air 10 Dec 1998
Ross – Birthday
In episode  his birthday is stated as October 18th.
Episode , which aired in October 1994, had Ross state his birthday was "7 months ago,"
which would seem to place it in March. And Ross told Gunther his birthday was in December, in episode .
Monica – About Age
In episode , "I’m not 22. I’m 25 and 13 Months" — Air 4 May 1995
That means In the first episode Rose was 26 and next month he would be 27 (If his birthday was the same as 902). And In episode  (May 1995) when Monica said she was 26 years and 1 month of age, Ross would be 27 years and 7 months of age.
Come to think about it, if you only see the first season. Ross was 26 in  then in  I know Ross’s birthday is in March that means Ross was 26 years and 7 months of age in  and he would be 27 years and 2 months while Monica was 26 years and 1 month of age in .
Have to calculate if want to know.
But it’s complicated because firstly I see Ross said 3 different birthday and I’m not sure when is Monica’s birthday. Secondary in the first season, I don’t think creators know exactly when the show gonna put on the air.
With all that I think about, my conclusion is Ross is an older brother of Monica.