Base on my experience, during flight training in Australia, my flight instructor was accumulating hours for ATPL, he only got into Qantas on the 2nd attempt.
My other flight instructor happened to be a Singaporean tapped on his networks in Singapore Airlines, couldn't get in, went to Australia to become flight instructor and cannot get into Qantas. The chances are very slim because when you look at the vast number of Flight instructor accumulating hours waiting to get a seat in Qantas.
As for nepotism, my friend back then when I knew him as a cadet pilot (now A350 Capt) told me while waiting for his interview, he saw a Captain and a person in formal wear walked in, opened the door. The Captain was greeted by name by quite a lot of interviewers in the room. The Captain put his hands around the person's shoulders and said, "My nephew ah".
I'm not saying SIA is a bad company. Nepotism exist in all industries.