Younger sis dating her lecturer?

I am sad - but I feel that I have no right to be. My younger sis - much younger than me - by more than 10 y - just finished her bachelor's degree and now is dating her ex lecturer. Which is OK - but he is double her age and married with kids - and openly admit he has no intention of leaving the wife. He puts her in an apartment near where he works. Mum accepts him with open arms. I just can't get over this feeling of sadness - and failure - as a big sister. Yes yes I know I should mind my own business. But I'm soo... twisted now. Arghhhh.... What do you all think?