Is there a way of completing Sjoring Hard-Heart quests without killing any theif members ??

Yes there is. You see, there is corruption in the Fighters Guild. The Balmora and Vivec branches are working with the Cammona Tong to get rid of the Thieves Guild.

What you must do now is go and talk to the head of the Fighters Guild in Ald-Ruhn. (Percius Mercius I think his name is, or something to that effect, he is an imperial.) You should be able to talk to him about this quest and he will tell you what to do. He can be trusted.