Your wife's daughter should be able to apply family visa or ILR(indefinite leave to remain) as her mother will be British citizen or currently ILR. Your wife has to provide evidence for why now and not before to bring her daughter to the UK. As you mentioned that her mum(your wife's mum) has passed away and nobody else can look after your wife's daughter. You need to have sufficient funding to support your wife's daughter as well.
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Information of how she(your wife daughter) can apply for ILR.
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