> > Is it
> > I have two lovely sisters, who(?) studies at the university.
[quoted text clipped - 6 lines]
>
> I have two lovely sisters who study at (the) university.
And the reason is: "Who" the relative pronoun can be either
singular or plural, depending on the grammatical number of its
antecedent. In mUslKa's two sentences, the antecendent of "who" is
"sister" in the first and "sisters" in the second. The antecedent
of "who" in the first sentence is singular, so "who" is singular, so
the verb is singular. In the second all are plural.
Note my careful eschewing of such a snotty remark as "I have no idea
why you have two lovely sisters studying at the university."

Signature
Bob Lieblich
Who hath no sisters, at university or otherwise