As a priest, may I advise you to sit down with the priest who has the cura animarum for you and your family? The priest who actually has your spiritual care.
**I find it very regrettable, frankly, that people seek advice on an anonymous internet forum about issues of such import that call for a physician of souls. Just as true, this is not a matter one should blithely answer. There are profound ramifications in terms of your relationship with your sister and her future husband, other members of your family, and their relationship with you. Actions of this nature can also further drive someone away from the Church.
If one had need of counsel about an issue of mental health or physical health, would one submit the case to anonymous people of dubious expertise? **
Yes, it is true that the priests of the SSPX have no faculties, neither by law nor by grant, whereby to validly witness marriages. (Pope Francis has said that a Catholic who confessed to an SSPX priest would be validly absolved, which he can do since he has direct and immediate jurisdiction over each individual.) Of course, a marriage in the chambers of a Justice of the Peace would also be invalid, if attempted by a Catholic.
The question of validity vs. invalidity admits a straight foreword answer. However, your question actually is whether or not you should attend; this requires a more elaborated analysis than simply whether or not the ceremony has validity in the eyes of the Church. And that is a question that requires being worked through thoughtfully and in an informed manner…not as a question and answer on the Internet.