If you are being interviewed for a job and find out afterwards that a member of the interview\hiring committee had a past romantic relationship with another candidate what should you do?
As a candidate, the best answer is probably: do nothing.
You don't know for sure that there is an impropriety. It's not clear that anything unethical or against institutional policy is taking place. At my institution, for example, those on a hiring committee are required to disclose any personal relationships with candidates, and make a declaration that this won't affect their decision.
There isn't an automatic policy of removing someone from a committee because of such conflict of interest. This is often a good idea, but it's not always possible. Some committee members may be essential for organizational reasons. For example, the head of a research group that is hiring a new member would rightly insist on being integral to the hiring decision. They can't be removed because of an issue like this--nor can people with a personal relationship to committee members be stopped from applying for a job.
In the real world, this sort of thing happens, and it can't always be avoided. It may be that everyone already knows of the relationship and has decided this is not a problem.
It's not clear that this would affect your chance of getting the job. Sometimes a personal connection unfairly prejudices hiring decisions. Sometimes people strive to avoid letting this prejudice their decision. And for something like "previous romantic relationship", it's not even clear whether the prejudice would be positive or negative.
You don't really know whether or how this affects your chance of getting hired.
Reporting is more likely to hurt than help your candidacy. To report it, would be to assume bad faith on the part of at least one person who is making a decision about hiring you. Reporting it would likely give you a bad reputation in at least that person's eyes, and maybe others as well.
If everyone already knows about the issue, your reporting comes across as questioning the work of the whole committee--not a good thing!
You could report it and hope to remain anonymous, but there's no guarantee of that.
Really, there is nothing unusual about the situation. Even though protections are put in place, hiring is almost never a level playing field. Very often, internal candidates have a leg up on everyone else. Advertised positions that are actually earmarked for a predetermined candidate are common.
This is unfortunate, but it's also reality. And because hiring decisions are always largely subjective, there is only so much that can be done about it. I don't think the information you discovered makes this situation unusual in any way, really. Try not to worry about it, do your best, and move on if you don't get the job.