My younger sister got married when I was 21 and she was 19. I never for a second thought that it was my right to tell her not to start dating. Who am I from preventing her from getting married? Why should I have that power? It is Hashem alone who decides who will get married and when. I did not feel jealous or bad that my sister got married first; what is meant for her in life has no connection to what Hashem has in store for me.
I don’t understand the concept of one sibling ‘not allowing’ another to start dating. To me, that reeks of immaturity and selfishness.