I think the maid have too much freedom to get the drink/food easily.
At the very beginning , I will show & tell the maid what should do, not allow to eat food inside freige except my permission. I also show her the daily schedule to follow the job on time. I am so serious & don't let her think that she is so important even both of you go out for work. Let her know that she just the helper to reduce the workload at home but not do her own way. Must obey the instruction by mom or sir. You must be hard & treat her strictly at the beginning. Now I think you better give her warning letter first, let her know that how you dis-satisfy her job & give time to improve, otherwise give her one month notice (of course in safety condition) & find another one. Good luck to you la
Have read your post some while ago, and truly pathetic about your situation back then.
For the time being, I don't have much suggestion but there seems still one remianing selling point of this Yan, and that is, she treats your girl not too bad.
Perhaps try those ideas suggested by moms above and see how it turns out to be. It can be understood that some can show improvement when warned, and some of them not. So it depends how much she takes your warning/persuation. If her negatives still persist, it is unavoidable to find replacement, though I think that her situation isn't something life threatening to the extent that calls for immediate dismiss.
To me, life with maids had never been easy for me, simply becuz i don't have the luck.