...Thing is, it's established pretty early on that the plane crash in question happened like a decade or more before the events of this game even start, so "recognition" isn't really a factor.
-Mal
Well, I went to a school reunion & hadn't seen most of those people for over 35 years.
It wasn't hard to figure out who most of them were, they recognised me, too & I'm not related to them.
If one of them was my Sister, I guarantee I'd know\recognise something about her features, her mannerisms, the way she stands, walks, moves, her voice.
I might not instantly place her as my Sister, but she would most certainly carry an air of familiarity.
Like when you meet someone & feel like you've met them before, but tenfold.

The post specifically said that she
wouldn't recognise him
because she has "memory block".
Therefore, using basic deductive reasoning, if she
didn't have "memory block", then she quite obviously
would recognise him.
In that case, it stands to reason that he would also recognise her, as he doesn't have "memory block".