You are 100% correct being with Marcel is inappropriate. Rebekah is the one who said Klaus loved Marcel as a son. She is also the one who wants to sleep with him. Even with vampires you dont bang your sisters or brothers or niece or nephew.
So let me start off with saying welcome to The Originals! I and plenty others in the TVDU consider The Originals is better than TVD and trust me it is but we leave that to you to find out. I would say it’s more mature than TVD. The thing with Mikael is Esther made him a true Original first. He’s a little enhanced more than the rest of family and also remember Mikael was a warrior, a Viking so he was very skilled fighter and warrior and his kids feared him! They were no match for him and at the time they didn’t knew their strength and what they were capable of so yes they all feared and ran from him. Just like any kid fearing they parents especially their father was a domineering alpha type father 😊
I'm pretty sure there's supposed to be a gap of time between when Marcel became Klaus's ward/adoptive son to when he became an adult where Rebekah wasn't with Klaus and Elijah, so I just hold onto the fact that she wasn't in his life during his childhood to make the relationship a little less weird :/
Rebekah last episode im getting Elijah back no matter what it costs. Klaus gets Marcel to offer Elijah back... You disgust me.... Rebeka is just mad she didn't play a bigger role to be a savior to Elijah.