Is it okay to jump in other people business

I have jumped in someone elses business, because It was my cousin and her boyfriend was beating up on her and I was not going to stand around and watch him continually beat her until one day he kills her. I think it is necessary to jump in other's business if and only if you know what is truley going on in tha person's life that is causing harm. I think people should mind their own business all the times, most people don't want you all in their life, if they want you to know what is going on they would come to you and let you know the problem is.