Is a step-child regarded as a family member?

[DavidR98]

Would I be able to sponsor an application for ILR made by my wife's daughter, at a time when she (daughter) is aged 18.

At present my wife could apply for FLR (M) and daughter (now 16) would be included in the application. (We have all lived in the UK since 206, but they hold visas as student and student dependant: I am a UK citizen.) However, once the two years is up enabling them to apply for ILR, she will be 18 and so too old to be included in her mother's application. She will be financially dependent on us and so eligible to apply using form SET (F), but at that point her mother will not have ILR and so will not be able to sponsor the application.