Is it ok to let someone use your account

true

This is clearly cheating.

It is not OK to let Magnus use your account. It is not OK to let your kid brother use your account. It is not OK. It violates the terms of service.

Yah. Cheating... But in hand and brain, the hand has to pick the piece to play. e.g, I'm the brain and I say 'pawn' the hand picks which pawn to move.

But allowing someone to play on your account is cheating.

ya that is cheating

Only if it was a shared account to begin with or if you both are on the same level for example both are around 1500.

clearly cheating because if that person who is using his or her account they might decrease there rating for any variant or so. all accounts should be used by its owner

Only if it is on chess.com :P