The answer is
the joker (playing card)
Here is the justification:
Contempt of court? I've done no such thing.
The joker is often removed from the deck, while other members of the court (i.e. kings, queens, jacks) are allowed to stay.
I have been part of this court for awhile now,
The joker has long been included in the deck of cards.
And all you have done is counted me out and laughed at me.
The joker is often removed ("counted out") from the deck. Further, a joker is someone who is laughed at.
Even worse since my brother left, you have resorted to calling me old and treating me as a servant.
Some decks contain two joker cards. Further, when playing Old Maid, you remove the second joker and let the remaining one play the role of the old maid, a servant.
I've had enough of this!
This seems like a filler line.
You want to get wild?! I'll get wild and trump all of you!
The joker is often used as a wild card that can become anything, including the trump. The appearance of the words "wild" and "trump" so close together was a pretty obvious giveaway of this riddle.