Baptized Catholics are bound to follow the rules in Canon Law regarding the form of marriage in order to contract a valid marriage. One of those rules is for the wedding to take place inside a Catholic Church. For legitimate reasons, this rule can be dispensed with by the proper authority (i.e. the local bishop).
If a baptized Catholic contracted a civil marriage outside the Church without a dispensation, that marriage would not be valid. Such a person would simply have to file some paperwork for “lack of form” rather than going through the full annulment process. Then they would be free to marry inside the Catholic Church.
How this all relates to your sister is outside of my purview to ascertain. If you think it would be important to her to be right with the Church in regards to her marriage, you could tactfully bring all this to her attention. If she’s not practicing the faith, then it’s less likely she’d be interested in following the Church’s rules.
If you have questions about how to handle it, it would be good to ask your priest for some advice.