they have the same mother. then they are half brothers and sisters. no the shouldn't marry. having similair genetic make-up will increase the chance of a bunch of genetic disorders and mental retardation.
[4.22] And marry not woman whom your fathers married, except what has already passed; this surely is indecent and hateful, and it is an evil way.
[4.23] Forbidden to you are your mothers and your daughters and your sisters and your paternal aunts and your maternal aunts and brothers' daughters and sisters' daughters and your mothers that have suckled you and your foster-sisters and mothers of your wives and your step-daughters who are in your guardianship, (born) of your wives to whom you have gone in, but if you have not gone in to them, there is no blame on you (in marrying them), and the wives of your sons who are of your own loins and that you should have two sisters together, except what has already passed; surely Allah is Forgiving, Merciful.
[4.24] And all married women except those whom your right hands possess (this is) Allah's ordinance to you, and lawful for you are (all women) besides those, provided that you seek (them) with your property, taking (them) in marriage not committing fornication. Then as to those whom you profit by, give them their dowries as appointed; and there is no blame on you about what you mutually agree after what is appointed; surely Allah is Knowing, Wise.