most likely he cant. it was said in FF7 that Sephiroth is the ONLY person who can use that sword.

In what game? FF7? Crisis Core? Dissidia? Kingdom Hearts? He's in more than one game, so the best answer I can give you is look up an IGN walkthrough or something.

'Mother', used by Sephiroth in the FF7 game/Crisis Core game and Kadaj, Loz and Yazoo, in FF7 Advent Children movie, basically refers to JENOVA, a calamity that fell from the sky and was mistaken for an Ancient (i.e. Aeris).

In short:

Mother = Jenova

It never says why, only that Sephiroth is the only person who can wield it.

No, 'Before Crisis' is a mobile phone game set 6 years before the events of the original Final Fantasy 7. It concerns the Turks, fighting against AVALANCHE.

'Crisis Core' is a PSP game, the events portrayed within are set a short time before the start of Final Fantasy 7, and mostly concern the backgrounds of Zack Fair and Cloud Strife.