My aunt was a Columbia admissions officer till a few years back. Will mentioning it help me get into Columbia?

Since nepotism is something actually looked down up at all elite schools I would highly recommend not doing this. Don't confuse nepotism with a development candidate (child or a wealth donor already contributing $$$) or a V.I.P. candidate (someone that is a celebrity or a direct friend of the Dean).

Plus you are an International candidate so what they will read into this is that you are outside of the US but have an aunt who used to work in the admissions office so you most likely do not have a close rapport with her or have any direct connection to Columbia. This will potentially do more harm than good.

No it almost definitely won't—and I imagine it would have a better chance of hurting your chances than helping them to mention it, unless you were to do so very specifically in the context of how it got you interested in Columbia (e.g., if you have any memories with her surrounding the school). But if you're thinking about it as a hook of some sort, it's not going to work like that in practice, and I can easily imagine it coming off to them as shallow.

I can't imagine this helping, especially since it sounds like she hasn't been an AO there in a few years. Really the only way I can see this benefitting you is by subtly working it into your essay (I'm talking like 1-2 sentences) or your interview. And that's only if you have some great memory with your aunt while visiting Columbia that made you interested in the school like @jcdenton said