I insisted on speaking to my girl in Mandarin at home and encourage to speak and reply me in mandarin as well. Her Chinese language teacher also provided guidance and told us encourage her to converse in mandarin, expose her to Chinese books and game play i.e. To the supermarket and teach her fruits and vegetables in English and Mandarin. Use life size item i.e. Bicycles parked at the void deck and encouraged her to point and count in Mandarin. Identify shapes and sizes in Mandarin too. Keep trying, encouraging and exposing to Chinese educational stuff and converse in Mandarin helps a lot. Avoidance results in reduced exposure. Whenever I asked in mandarin and she replies me in English, I will correct her and remind her to reply me in mandarin.
This may seem like a cliche, but every child is different. What happened with your nephew may be worrying, but still, it may not happen to your child. That said, Singapore's larger environment tends to encourage English more. I would certainly speak more mandarin to my baby because this period is so crucial. For example, sing to baby. Mommy and daddy's voices are the most beautiful things in the world to them. I used to be able to stop my son's crying just by calling his name : )
Arghh im Struggling as well. You. Are not alone. Guess what, my boy refuses to speak mandarin to me. When i speak to him, he will give me the blur look. Really out of my wits ..