I am here with my third theory;And yes I do think so that Ben and Gwen are Sibling.

Want to know why I think so;-

MAX TENNYSON

CARL TENNYSON FRANK TENNYSON

BEN TENNYSON GWEN TENNYSON & KEN TENNYSON

There are no arrows try to make sense of this chart.

In 1998 there was 1.2 % increase in population.OR lets say there is a city A with 1000 total population in 1997 then it would grow to 1012 people in 1998.

As Carl and Frank are only known child of Max so finding out probability of Ben and Gwen being born in same year to respective father would be;

1000/500=2 -------> which is number of Max's child

So, 1.2/500=0.0024%------> chances of being born in same year

And chances of there being born on same date are

0.0024/365=0.00000657534%

Yes i know it says that

It would take 657534000 brothers to have 1% chance of having babies on same day.

NOW FOR TWIN THEORY

Chances for twin pregnancy are 3 in 1000

Process same as above;-

1000/500=2------>Number of Max's child

3/500=0.006%

And as twins are born to single mother so we do not need to find chances of each mother.....

HENCE;;;;;; Chances of

GWEN AND BEN BEING TWIN are 1000 times more than being cousins and sharing same Birthday.

Please tell me about your thoughts in comments