How to say "oink!" in Mandarin in a family situation?
Well, the porcine vocabulary comprises at least 3 sounds. How one may represent them is a matter of choice. To date, no exhaustive linguistic studies, to my knowledge, exist.
'oink' is phonetically interesting, because the sound is produced by inhaling. To mimic the 嗷嗷 grunt adequately, one should also inhale.

grunt: 嗷嗷，呼噜噜
squeal: 吱儿吱儿
oink(somewhere between grunt and squeal): 哼哼, 耶耶

当然，不同地方的猪叫声是不一样的， 因为各个地方的方言不一样。
东北的猪叫声听起来和广东的不一样。