Honestly if this is the rule of the game, i don think there is anything you can do, esp she is taking care of your old parents on her own, you won dare to upset her too much. Unless she deliberately do something that hurts yr parents, I don think you can fire her without compensation. Pls also noted that her cost for delivering the baby in HK is same as normal HK residents as she is a HK ID holder but of course she has to bear the cost to take care the baby.
If you get rid of her now, you need to pay her
1) 3 months salary until her delivery (assume she is 6 months now)
2) 10 wks m leave but you can offset the first 4 weeks with her last month salary, i.e. extra 6 weeks
3) 1 month notice if you file her after m leave
4) 7 days AL
5) airfare
I suppose the total of the above should be ~20K ony instead of 50K as you said. or may be you can bargain with her for a lesser amt to break up.