Cecille is a cunt and she will do as cunts do, be a cunt. It's on Erin for not being man enough to say no and maintain his own boundaries.
And once again it comes down to Erin's inability to manage his boundaries. He doesn't feel the same way Sera does but he does like her enough to continually lead her on and get mad when she eventually moves on. He should have either put his foot down and taken control of his own relationships Cecille be damned, or cut Sera lose in a humane way before leading her on her entire life. Not discounting how much is Cecille's fault, but Erin's inability to say no and to be walked all over like a dickless simp is entirely on him.
It's been pretty well confirmed that all of Mars' main girls, including Erin, are going to have generally positive relations with him once the harem path and individual paths open up. Maybe people who're excluding Erin from their harems will get him a bad ending but as I understand fem Erin is all in on his relationship with Mars.
Erin doesn't know how to maintain boundaries nor is he even mature enough to understand that. It's not his fault he is coddled and didn't develop like most people do. His father died either before he was born or shortly after and as we can see, his mother is very dotting, so he didn't have a father or disiplinary figure that we know of.
I don't think Erin could cut off Sera without her killing herself (unless what happened happens). She has proven she would die for Erin and it doesn't seem like Erin understood that nor was it really communicated.
Similar logic could be applied to Cecille, she grew up worse than being adopted and her mother was a prostitute. I think she's a sociopath because she is still emotional. Sociopaths are made during childhood, not born. The main difference is that she knows what she was doing was wrong and would turn out shitty so she could reasonably be guilty.
All of that to say, how much can you blame someone if they can't comprehend it?