And of course I confused this with Spanish and put "she is us" before I noticed that didn't make any sense and lost a part of my bar :L
Ser = Sees, happens to spot. "Han är på promenad och ser en ko." "He is on a walk and sees a cow."
Tittar = Looks. "Han tittar inte." "He is not looking." (i.e., he is for example covering his eyes.)
Tittar på = Watches, looks at something. Must have an object, though it can be an implied one. "Han tittar på tavlan." "He is looking at the painting." "Ebba spelar spel och Kalle tittar på." "Ebba is playing games and Kalle is watching ("Ebba playing games" being the implied object)."
doesn't make sense in english, it would be "She is looking at us." or "She sees us."
We have an expression in English. "She is seeing him." means that she is dating him.
True, though that expression does not translate literally to Swedish. "Hon dejtar honom." ("dejtar" being a "Swedishfied" form of the English "dates") or "Hon är ihop med honom." (Literally: "She is together with him.") would be legitimate translations to your sentence.