Why can Romeo date Roseline and not Juliet if they are both of the Capulet house?Rosaline is listed on the invitation as Capulet's niece.

Actually, in Shakespeare's Romeo and
Juliet, Rosaline is a member of the Capulet family (as
mentioned) and, in this case, is Capulet's niece. This is
an excellent question.

I don't believe there is
a way to know the "correct" answer—or even if there is one—but I
would surmise that there are probably several reasons why Romeo's feelings are not the
cause of upheaval.

In Act One, scene two, Romeo chances to
meet Capulet's servant charged with inviting guests to the party, but he cannot read the
list. He asks Romeo for help. Romeo reads the list:

readability="12">

Stay, fellow; I can
read.

He
reads.

‘Signior Martino and his wife and
daughters;
County Anselme and his beauteous sisters;
The lady
widow of Vitruvio;
Signior Placentio and his lovely nieces;

Mercutio and his brother Valentine;(70)
Mine uncle Capulet, his
wife, and daughters;
My fair niece Rosaline and
Livia...

My first reason is
based mostly on conjecture based upon what little we know of Romeo's relationship with
Rosaline. It would appear that he simply "moons" after her, wanting her love, which is
not returned. There is no way to know that the relationship has brought them closer than
writing letters. Either way, there is no danger that anything will become of it as
she wants to join the Church. This association may not be
considered threatening to the Capulets.

Secondly, the feud
exists between the Capulets and Montagues, and we can certainly appreciate how deeply
the animosity runs between the two families when the servants are ready to kill each
other. However, Romeo presents no threat. He is not at all involved in the fighting,
only in "the loving." We also know that at least at the beginning of the play, Capulet
has no hard feelings toward Romeo when it is discovered that he is crashing the
masquerade party. Capulet is firm in denying Tybalt the chance to confront Romeo at the
party. In fact, Capulet finds nothing offensive in the person of Romeo or his uninvited
presence at the party, but defends him as a seemingly decent young man, in Act One,
scene five:

readability="43">

TYB:

Uncle,
this is a Montague, our foe;

A villain, that is hither come
in spite

To scorn at our solemnity this
night.

CAP:

Young
Romeo is
it?

TYB:

'Tis
he, that villain
Romeo.

CAP:

Content
thee, gentle coz, let him alone.

He bears him like a portly
gentleman,

And, to say truth, Verona brags of
him

To be a virtuous and well-govern'd
youth.

I would not for the wealth of all this
town

Here in my house do him
disparagement.

Therefore be patient, take no note of
him.

It is my will; the which if thou
respect,

Show a fair presence and put off these
frowns,

An ill-beseeming semblance for a
feast.

Finally, had Romeo
been pursuing Capulet's daughter, rather than a niece (and who
knows how involved her parents are in the feud), the situation
might have been very different.

(It is interesting to note
that, unless there is another Mercutio, Romeo's friend Mercutio—and his brother
Valentine—are also on the list of the invited. Mercutio is very good friends with Romeo
and everyone is sure to know this, but he is also Prince Escalus'
kinsman, and included despite the connection with the Montague family. The separation of
the two families may not be as clean cut as one might like while living in the midst of
a feud.)

Of course, there is no way to know for certain,
but I would expect that this line of reasoning would serve in trying to understand this
small contradictory element Shakespeare felt the need to include in his
tragedy.