Is there any situation where the ci here could have been for emphasis and not meaning "us" - like, "My sister buys this here cake" approximately in English? Just trying to get this "pronoun vs. emphasis" idea down...
Hmmm. Did you write 'Lui ci legge un giornale'? That is correct. On the other hand, if you wrote 'Lui leggi ci un giornale', that is incorrect word order: clitic pronouns appear immediately before the conjugated verb.
A noi = ci. In general that is the role of the indirect object pronouns: a me = mi, a te = ti, a lui = gli, a lei = le, a noi = ci, a voi = vi, a loro = loro. Their placement just infront of the verb is crucial syntactically.
This one could be even trickier, since "ci" could also mean "with it".and put an emphasis on the means of the action.
E.g. "Con tutti quei soldi, mia sorella ci compra una torta".
(="With all that money, my sister buys [even better, could buy] a cake")