There are various ways to say this. " If you had not told me it was Sarah, I would not have known which sister called.
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alc24So I say:
I could wonder which of the 2 sisters actually sent me the message because sarah isn't one to do so usually.
I could start to wonder which of the 2 sisters actually sent me the message. (if you hadn't told me it was Sarah)
If you hadn't told me, I could have wondered who sent me the message.
If you hadn't told me, I couldn't have helped