Is this selfish of them?

I have been offered a job in San Francisco because of my good work ethic here in London. I've been working for this company for 2 years now. I've moved back in to my parents 2 years ago because I was in debt and needed to get back on my feet. Anyway, I managed to save up and pay off my debts. Now my dad has cancer and my mum is financially unstable. (they are divorced) my sister is out of a job. This job opportunity is very rare for anyone in my company to get, you actually have to work harder than everyone else. My mum told me that I shouldn't go because I need to help her with financial problems and that I have to look after my dad. If I wanted to go, at least wait until my sister has a job. Is that selfish?