According to some sede websites, the rite of ordination was changed in 1968. These changes were so bad that they made ordinations invalid. Therefore, all priests, bishops, archbishops, cardinals, etc since 1968 are invalid. The Church teaches that only validly ordained priests can administer the sacraments. Based on this knowledge, it seems that all sacraments administered after 1968 have been invalid. Is this really what sedevacantists believe?

Sedevacantists believe what the Church has always taught.  Since the Church teaches that even heretics, apostates, and pagans can validly administer the sacrament of baptism, then sedevacantists obviously believe that baptisms administered by Conciliar priests are valid providing that the priest uses the proper form of the sacrament (which was not changed in the Novus Ordo), uses the proper matter (water), actually pours it on the head of the person being baptized (and not just on the hair, for example), and intends to wash away original sin.  

(Note that the Church teaches that proper intention must be assumed unless the priest gives positive evidence that he intends something different.  If the priest were known to reject the doctrine of original sin and proclaimed that baptism was merely an initiation ceremony, even most anti-sedevacantists would likely believe a baptism to be doubtful, at best.)

Think, man.