It is not advisable (genetically) for first cousins to marry. There could be many medical and genetic issues involved. However it is not assur.
As far as family is concerned, it is not so black and white and simple as one might think. If you come from a holocaust survivor background family is a huge inyan. parents, aunts, uncles, cousins, etc. are all part of a puzzle of quilt that grows and each piece is a very huge part and parcel of the fabric of the outcome. For cousins not to speak or not to get along is stab in the heart to the survivors. They want to gather as many as they can and see the results of their efforts and the affirmation of why they survived.
Like my mother a”h would say “you are NOT talking to my shvester’s knit?” you are not speaking to my sister’s child? That would kill her because her sisters children were just like her own.