No, it isn't a sister blockchain. It is a blockchain based on Avalon. It grabs the blocks from the Steem blockchain and processes certain instructions on the sidechain. For example, if you want to buy self-created tokens/NFTs or similar with STEEM. The possibilities are really very diverse.

The previous Steem engine had the disadvantage that it was not decentralised. It could therefore be switched off from one day to the next. This will no longer be the case with the planned sidechain. As with Steem, there are witness nodes that ensure the operation of the sidechain.

But I don't know which security issues you mean.