Solution:

A Balloon , It was Sonal's Birthday.

Solution:

BACK

All the others can also be read backwards as proper words.

Solution:

They all read the same rightside or upside down.

Solution:

Each word has 3 consecutive letters of the alphabet-
couGHIng (GHI)
thiRSTy (RST)
DEFiant (DEF)

Solution:

A Coin (has a head and tail)

Solution:

Neither, the accountant was his sister.

Solution:

In Washington D.C.

Solution:

7 up cans

Solution:

No time at all it is already built

Solution:

1st person : Colin
2nd person : Rooney

They both lied.(I said at-least)
(If only one lied they would both be boys or both be girls)