you need not worry about tax on money in S.B a/c.
on the contrary,money in savings bank earns interest @4% P.A.some banks pay even more.
the question of taxing money in savings bank does not arise.
as far as repayment of loan by you from out of your brother's savings bank a/c,,keep carefully all correspondence,records , receipts and documents for any future verification
.your brother needs to explain the transactions in his bank a/c,in case of any inquiry /verification.
being an nri,you can open NRE/NRO a/c,wherein you can put money in or draw money out of them or transfer or save as you like.
as far as upper limit of amounts kept in S.B a/c,there is no restriction, though some banks may not allow more than Rs 10 lacs.
but remember cash deposits in S.B a/c of Rs 10 lacs or more in a year will create problems U/S 285BA of ITAct.