I'll concede that it may seem fairly obvious... however, Animus hasn't said anything to that effect, and none of the NPCs who know have said anything, either.
Of course, Jirlai's offer to Animus may have been a little suspicious - why would Jirlai offer for Animus to come with to save the girl, when there is almost no relation between the two?
TFO - as for getting a sense motive check to see Animus' reaction... I don't think I like the idea of having players bluffing and sense motive-ing each other. If Animus responds to what you said, then - if, as a player, you think Ruth would be able to figure it out, I suppose I don't mind... IIRC, there hasn't been anything said about her being Animus' sister anywhere, so I don't think you're metagaming. Just remember that, if Animus gives a convincing argument, that you should really try to figure out what Ruth would think, as opposed to using what your OoC knowledge.
CS - do you have a problem with TFO knowing?