Man sleeping with mother as wife - custom

I don't understand the meaning of customs and tradition any more and i think if there is need Nigeria should be redesigned. You see, somethings sounds like taboo, abomination and incense or better still sacrilege. But, i dont even know these things existed, where it is normal for a man to bed his mother in the name of custom, until my relocation to my new station.
do you know in a part of Nigeria, a woman or widow is permitted to marry her son and raise (i mean live together, sleep together and conceive and bear children for him)children for him? Now how do you feel having to choose between your girl friend and mother?
Let me be candid a little. A woman's husband dies and she was being asked if who would she want to be re-married to? She simply said her first son against the brothers in-law eying her. the son accepted and they performed the necessary traditional marriage right and was allowed to take the so called wife (mother) to his bosom. The man keeps the new wife in their home town and comes from Abuja to visit as often. Bobbo just newly got married to fine girl but never explained the other side to the wife.
The height of the whole thing happened when both mother and new wife were pregnant for man.
The lady felt like committing suicide but was told that that was the custom and that actions was uncalled for.
I tried asking a friend why would such a custom be allowed but he simply asked me, what difference has this from men who marry their late brothers wives, in some part of the country.
For me, even if that was the only thing to reject that will make me being an outcast, i think it's worth doing. Some of these evil customs and traditions should be checked and be abolished.