This is the mixing that is allowed or not allowed by Allaahu Ta'ala:
4:22. And marry not women whom your fathers married, except what has already passed; indeed it was shameful and most hateful, and an evil way.
4:23. Forbidden to you (for marriage) are: your mothers, your daughters, your sisters, your father’s sisters, your mother’s sisters, your brother’s daughters, your sister’s daughters, your foster mother who gave you suck, your foster milk suckling sisters, your wives’ mothers, your step daughters under your guardianship, born of your wives to whom you have gone in - but there is no sin on you if you have not gone in them (to marry their daughters), - the wives of your sons who (spring) from your own loins, and two sisters in wedlock at the same time, except for what has already passed; verily, Allâh is Oft-Forgiving, Most Merciful.
Source:
http://www.qurancomplex.com/Quran/Targama/Targama.asp?nSora=4&l=eng&nAya=22#4_2224:31. And tell the believing women to lower their gaze (from looking at forbidden things), and protect their private parts (from illegal sexual acts) and not to show off their adornment except only that which is apparent (like both eyes for necessity to see the way, or outer palms of hands or one eye or dress like veil, gloves, head-cover, apron, etc.), and to draw their veils all over Juyûbihinna (i.e. their bodies, faces, necks and bosoms) and not to reveal their adornment except to their husbands, or their fathers, or their husband’s fathers, or their sons, or their husband’s sons, or their brothers or their brother’s sons, or their sister’s sons, or their (Muslim) women (i.e. their sisters in Islâm), or the (female) slaves whom their right hands possess, or old male servants who lack vigour, or small children who have no sense of feminine sex. And let them not stamp their feet so as to reveal what they hide of their adornment. And all of you beg Allâh to forgive you all, O believers, that you may be successful.[1]
Source:
http://www.qurancomplex.com/Quran/Targama/Targama.asp?nSora=24&l=eng&nAya=31#24_31
