They don't need to hurt her.
They could drug her, fuck with her mind to use as an inside agent or even keep her as a "special guest" until they get what they want.
You also have to realise how that would look to her. She goes there with him and he just leaves her behind without a word. If that was me i'd be having severe trust issues, I certainly wouldn't go anywhere with him again.
"Hey, guess what, I got my memory back sis" .... simples.
Doesn't need to trust her but it's not difficult to tell someone you were once supposedly close to that you remember who she is. It's not like he showed any inner turmoil over it.
Those people still show love to people they care about. There is a time and a place to bottle things, they aren't robots that walk around constantly stone faced.
He's barely out of that learning. He wouldn't have the experience nor reasons to bottle up all his emotions all the time. He's not an experienced field operative he's barely out of training and has issues with his memory.
With the possibility of Cam being held by the Ishi's, very doubtful, they need the meetup more than MC and his people do, they drug her, hurt her anything like that, and their chances of survival are nil, let alone their chances of doing a deal.
How Cam would react to his leaving is about the same, she arrived at the house alone, and the Ishi's will more than likely tell her he needed to go and talk to an important business contact, not knowing he was that person, they do anything that goes counter to that they lose so...anyway.
"Hey, guess what, I got my memory back sis", and now we are good... Nah, that wont work, she would be insulted by the fact that being his sister means he ignores his hunches, what if he does that again when his life or hers is in danger.
Trust will take time, agreed, but he lets his guard down to his ladies, heck he acts like he'd let one get away with murder, and that can be a weakness.
The last bit is pretty well covered below. Since I was saying in my original post that he needed to show more emotion at the right times.
To people arguing that the school taught him to conceal his emotions, there is a counter argument - the school is all about power, winning and control, showing emotion (even if fake) can be a very effective way of manipulating people. The school would know this and would teach this as it is a powerful way to get what you want (in certain circumstances). The MC is supposed to be good at subterfuge - emotional manipulation comes under this.
Your right he should have learned both to hide, and to show emotions as needed, which is part of what I meant by it being a difficult tradeoff.
MC needs to show his feelings more among his loved ones, yes, that includes Cam and all the other ladies, but he still needs to hide his emotions around those he doesnt know as well.