I don't speak from my own experience, but breast cancer can spread to form secondary tumours in the eye, though this is rare.
If your SIL has a "benign tumour" this is different from a secondary cancer.
I hope you will both have helpful advice, and that she will have the appropriate and successful treatment, soon.
My SIL has a benign tumour behind her eye which is pressing on her optical nerve. Luckily, she has private insurance which means that she has been offered the cyber knife to stop its growth. (My personal feelings it should be a treatment available to all but that's another discussion).
Has anyone had experience of the cyber knife? I'm trying to understand it all and the side effects that she may have in her recovery. She was so supportive to me following my dx and through my treatment that I'd like to offer her support but I just don't know what to expect.
Many thanks in anticipation