Women site, - How Advisable is Marriage Between Relatives?

Marriage is a means of establishing new relationships, and expanding
the sphere of connections within the community. However, marriage
between relatives does not contribute to establishing new
relationships as is the case in marriage to non-relatives. So, it is
better for a Muslim woman to marry a non-relative if there is a
potential husband who possesses the specifications that pleases her
and her guardians.
In many cases, to marry a non-relative is necessary if there are no
relatives available, who have the characteristics of the Muslim
husband that are stipulated by Islam, with respect to piety, a good
reputation, and a healthy body, soul and mind; or if there is such a
man but the woman or her guardians did not accept him. Getting married
to non-relatives provides the woman with a wider field of establishing
connections, as a result of exposure to an environment and community
other than the one she was brought up with.
Moreover, marriage between relatives might lead to weak offspring due
to the accumulation of negative hereditary characteristics. It may be
this factor which prompted 'Umar bin Al-Khattaab, may Allaah be
pleased with him, to advise the Muslims saying, "Marry from
non-relatives lest your )offspring( would be weak."
Of course, this does not mean that marriage to relatives is prohibited
or unfavorable, if the suitable conditions are met. Rather, this draws
our attention to the danger of preventing marriage to non-relatives,
as is prevalent in certain Muslim communities.