If your job duties are expected to have any overlap at all, I think it's reasonable and uncontroversial to veto the hiring of someone you know you aren't going to be able to work with productively. So that's a starting point.

But if there's no expected overlap in duties and you just don't like the idea of her being in the same office, it seems to me that that's not really a professional issue anymore. It's a personal problem. (There's nothing wrong with having personal problems, everyone's got some.)

It is a little unethical to negatively impact someone's career due to a personal problem you have with them, I'd say. But if changing your feelings isn't an option, it's hard to see that you have a choice other than telling your employer about your problem and suggesting a no-hire.

I'm going to push back on this otherwise excellent answer a little bit and raise the issue of company culture. While personal issues shouldn't--in the ideal sense--impact the workplace, knowingly bringing them into the fold could negatively impact the company as a whole. Be it frosty relations between two employees causing others to take sides and create division where none existed previously or direct confrontations that interrupt all other parties.

All hires bear risk that they won't fit it in or they'll have a hard time adapting to company culture. Knowingly bringing in someone with strong personal issues with another employee should require a higher level of scrutiny to ensure the issues won't result in a toxic workplace or destroy internal culture.

Reasonable answer. Everyone's job seems to overlap a little at such a small company, but for the most part no, they don't overlap.... I think I've resolved to simply wait until my coworkers ask me about working with her (should happen at some point if things happen to go well for her), then be honest and say how we don't get along, how it would be a negative influence for me.

I would let the hiring manager know that there are issues between you and your sister. You probably don't need to elaborate beyond "family problems" She doesn't have the job yet, but that at least lets him or her know that there are things to watch out for.

If they do hire her, just be professional. If your sister cracks and starts being unprofessional, then it's not your fault.

The best revenge is living well.

Tell the hiring manager that you are uncomfortable with the idea of working in the same workplace as other family members.

No more explanation should be offered or provided.

Be aware that your expectation that your coworkers would talk to you before contacting your sister was rather unrealistic.

Also be aware that that this is essentially "she goes or I go." Take the steps necessary for the latter situation. It's a business and who to hire is a business decision.

Nobody else can help you with this.

What do you value more, your happiness or your sister's happiness? That's what it really boils down to. Nobody can answer that for you, and nobody else should.

If she is actually professional and good at what she does I'd be more inclined to let it go and see if they hire her. If she starts acting unprofessionally after she starts then I'd raise that like you would any other employee.

I wouldn't just tell them to not hire her, but I would voice your concerns. If they think that she is the best fit for the job still, then suck it up and deal with it.

Just tell them straight up if they hire her you're out.