In the “Help me, Obi Wan Kenobi, you’re my only hope” hologram, when Obi-Wan first saw it, did he know that it was Luke’s sister?
Consider the following:
Obi-Wan knew that Luke’s sister Leia had been left with Bail Organa, since he was there when Bail took her. From the transcript of Revenge of the Sith:
OBI-WAN: We must take them somewhere the Sith will not sense their presence.
YODA: Split up, they should be.
BAIL ORGANA: My wife and I will take the girl. We’ve always talked of adopting a baby girl. She will be loved with us.
Obi-Wan knew that the message came from the daughter of someone whom he had fought with during the Clone Wars, and who lived on Alderaan. From A New Hope:
LEIA: General Kenobi, years ago you served my father in the Clone Wars. Now he begs you to help him in his struggle against the Empire. I regret that I am unable to present my father’s request to you in person, but my ship has fallen under attack and I’m afraid my mission to bring you to Alderaan has failed. I have placed information vital to the survival of the Rebellion into the memory systems of this R2 unit. My father will know how to retrieve it. You must see this droid safely delivered to him on Alderaan.
Finally, whoever the person was knew both his real name and had a general idea of where to find him.
With these pieces of information in mind, it would have rather odd had he assumed it to be anyone but Leia Organa.
In ANH, he did not. Lucas did not have ESB, or ROTJ written yet. Luke and Leia were not related in any way until ROTJ was written, hence the brother and sister kissing thing. The original intent was that Leia was a princess and rebellion leader only. Luke was to have a twin, and the original concept was to have him searching for, and finding them in the next 3 episodes after ROTJ. Not sure why that was all scrapped. Lucas did some retconning in esp. 1, 2, and 3. Unfortunately it created other continuity errors with the originals. This particular time, Obi-Wan was not lying or telling things from a "certain point of view". The script had Luke and Leia not related at all.
Here is one link I read some of that at.
Here is another on Twitter from Carrie Fisher herself. Pretty much confirmed that the twins thing with her was not planned.