Using NBitcoin or C# to derive Litecoin and Ethereum Addresses from Master HD Key
Not sure how NBitcoin works, but it looks like so far you have derived your private and public key, you now need to hash it in the Ethereum method rather than the bitcoin way.

Look at step 3 in this answer: https://ethereum.stackexchange.com/questions/3542/how-are-ethereum-addresses-generated

Edit:
Nethereum has implemented it in C#: https://github.com/Nethereum/Nethereum/blob/master/src/Nethereum.Signer/EthECKey.cs#L115