Salamun Alaikum,
Marriage into 'families' as you have advanced, is not prohibited by the Quran nor is it expressly encouraged. It is a matter simply for individuals and communities based on sensible choices.
The Quran only stipulates boundaries pertaining to relations that cannot be married (4:23) These do not include first cousins which would remain permissible.
I cannot speak for every community, culture or religion but such marriages are / have not been, exclusive just to Islam.
I hope that helps, God willing.
Regards,
Joseph.