Dear Sir,
Firstly, I don't know but I felt after reading your query that there is cofusion regarding in whose hands the gifts are taxable.
Gifts given by you will be taxable in the hands of recipient.(if at all they are taxable) - So no tax liability in ur hands...
Now coming to ur specific cases -
1) Gifts of any amount received on the ocassion of marriage of the individual are exempt. -As per this clause the sum will be exempt in the hands of ur nephew & his wife.
2) Giftsof any amount made to specified relatives (Brother is one of them) are exempt irrespective of the ocassion - As per this clause the gift will not be taxable in the hands of ur Brother.