Is the marriage considered void if I swore that I would seek a divorce from my fiancé if I did something, then I did that thing and did not expiate my oath, and is it permissible for me to expiate this oath at any time, even if it is after the marriage?

If the sister uttered an oath regarding a matter, then followed it by saying that she would not accept such-and-such if such-and-such was done, then she is considered to have sworn an oath, and the breaking of the oath depends on her if her oath was binding. If she breaks it, then expiation is due upon her. Fulfilling the oath may be obligatory if the oath was to abandon something forbidden or to perform something obligatory. As for conditioning the non-acceptance of a suitor or requesting a divorce from him if he does something, nothing is entailed by it, and it does not prevent the performance of the permissible act.