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1.
Siblings who are constantly mistaken for twins.
I went out with my twinster Sunday night and everyone did a double take.
by Conan O'Barbarian July 29, 2010
 
2.
A twin brother or sister who become pissed off at one another.
Connie unjustly accused her twin brother brother Ronnie of some nonsense that caused him now to refer to her as his twinster.
by Twinster April 09, 2008
 
3.
Identical or non identical birth twins that have severed their his or her family relationship for one reason or another.
Ronnie now refers to his estranged twin sister Connie as his Twinster.
by Twinster April 09, 2008
 
4.
Identical birth twins or non-identical that have severed their family relationship for whatever reason or reasons.
Because Ronnie and Connie twins had a very serious falling out, he now refers to her as his twinster.
by Twinster April 09, 2008