The Lightning network is not a sidechain.

A sidechain relies on its own blockchain which is coupled to the Bitcoin blockchain via a two-way peg.

On the other hand, the Lightning Network consists of native Bitcoin 2-of-2 multisig transactions.

When two Lightning nodes open a payment channel, they both send funds to one multisig address and each provides the counterparty with a pre-signed exit transaction. If a party wants to resolve the payment channel, they can unilaterally add their own (the second) signature and send the transaction to the Bitcoin network for confirmation.