Bitcoin is too inflexible for what Ethereum is trying to do and vice versa. With Bitcoin Ethereum has a safe and reliable store of value which it can integrate into its systems. With Ethereum Bitcoin has a platform uniquely suited to expand its potential. It's a win-win situation.
When and how did Ethereum solve the two-way-peg-problem that is required for it to "use Bitcoin as a reliable store of value which it can integrate" ?
Ethereum will not work like a sidechain. Distributed apps running on ethereum will run Bitcoin wallets and operate them through smart contracts.... I think.