From what I've been told, It shouldn't be a problem as long as he has adopted her legally Otherwise there is no legal relationship with her that would allow her to be his legal dependent here. I may be wrong and maybe somebody else here has some better information as I cannot provide a link to support this.
This is the closest I've been able to find thus far but it geared more towards adoption and less towards a stepchild from a spouse's previous marriage.
https://www.msf.gov.sg/Adoption/Pages/A ... -Pass.aspx
This from an earlier post on this forum from 4 years ago....
viewtopic.php?f=78&t=101288&p=683952&hi ... vp#p683948
This from another expat forum some of us used to frequent a dozen years ago before it went sour. This thread is 12 years old but I don't think anything has changed since then. It says pretty much what I've said and the previous link says....
http://www.expatsingapore.com/forum/ind ... #msg222336
Another regular here, PNGMK, may have more insight here with that regard as I think he has adopted while he has been here (he's a long-timer like my self with over 25 years here).