Why does Lady Capulet think that Benvolio is lying to the prince when he tries to explain the fight that happeneded between Romeo and Tybalt?

Since Lady Capulet and Tybalt are blood relatives, she
would obviously, out of loyalty, defend his honour and be less believing about what
Benvolio reports. Although he reports as accurately as he can, it is the terms that
Benvolio uses when referring to Romeo's part in the fracas that makes Lady Capulet
believe him even less.

Benvolio
states:

readability="11">

"Romeo that spoke him
fair, bade him bethink
How nice the quarrel was,
and urged withal
Your high
displeasure: all this
uttered
With gentle breath, calm look, knees
humbly
bow'd,"

He is
saying that Romeo was not aggressive at all and used gentle, conciliatory terms to calm
Tybalt down, even telling him about the Prince's displeasure about fighting in the
streets. Romeo was humble, calm and gentle in his manner. Tybalt was, however, "deaf to
peace" and attacked Mercutio,  stabbing him when Romeo
intervened:

readability="5">

"...underneath whose arm
An
envious thrust from Tybalt hit the
life
Of stout
Mercutio."

Tybalt
then flees but returns later. Romeo knows at this point that Mercutio has died and
he has sworn revenge. He fights with Tybalt, killing him. Romeo then
flees.

Lady Capulet obviously does not believe Benvolio's
version of events, saying:

readability="7">

"He is a kinsman to the
Montague;
Affection makes him
false; he speaks not
true:
Some twenty of them fought in this black
strife,
And all those twenty could but kill one
life."

She
suggests that because Benvolio and Romeo are related he would, of course, lie. She
states that the brawl had been a cowardly act and that Tybalt had been outnumbered
twenty-to-one. It took twenty of them to kill Tybalt, suggesting Tybalt's bravery when
he was grossly outnumbered. It is ironic that she takes this stance even though she had
not even witnessed the encounter.

A further incentive for
Lady Capulet's disbelief could be that, since she knows that the Prince and Tybalt are
also related, that he would take her side, which the Prince does. He queries who should
be punished for Tybalt's death and ignores Lord Montague's suggestion
that:

readability="9">

"Not Romeo, prince,
he was Mercutio's friend;
His fault concludes but what the law
should end,
The life of
Tybalt."

In
other words, "a life for a life" - the score had been settled. The Prince commands
that:

"And for
that offence
Immediately we do exile him
hence:"

and:

readability="9">

"I will be deaf to pleading and
excuses;
Nor tears nor prayers shall purchase out abuses:
Therefore
use none: let Romeo hence in
haste,
Else, when he's found, that hour is his
last."

He
clearly displays favouritism.