hi there
i had sent the bitcoins from kraken ac to my sister old address in coinbase which was generated 3 weeks ago, she received nothing, but i could see the transaction on blockchain went through and the coins are there unspent.would i get them back automatically
No. You sent them to an address. They went to the address. That's how bitcoin works.
or i need to contact blockchain?
Blockchain.info runs a website that tells you what they see on the bitcoin network. There is nothing they can do to change what you did.
please advice me how to recover those back.
regards
If that address is an address from Coinbase.com, then Coinbase.com has the bitcoins. You can contact them and see if they are willing to do anything to help you.