[quote TakeYourFinalPosition]@purplepots I'm glad they've got support, and I hope you have, too. I'm so sorry for your loss 
Citizens Advice will usually not give advice on immigration issues; and it's a really complex area of law, so it's typically not covered at free legal clinics either. It's very likely that they'll need an immigration specialist; and they're likely to be looking at a bill of a few thousand to attempt to do this - so that might be something to chat about as a group first, to see if there is any way to fund them coming over.
I'm not an expert and nobody would be able to give specific advice here as it's too dependant on details; but in general, it doesn't sound like she'll qualify for a spousal visa, and it's unlikely that his step-daughter would be able to come over. His step daughter might be able to study here as an overseas uni student; if she has the qualifications/income. His wife might be able to get a skilled persons visa, if she has the required skills/experience.
His daughters will be able to apply for nationality, but not necessarily citizenship, unless they could come and live with someone resident here. Being the parent of a British national doesn't usually provide a visa, and it sounds like they're too old to use the school child route.
It's a long and expensive journey; and while I'm absolutely not saying that it's not the right thing to do, it might be prudent to give them some time to adjust and decide what they'd like to do before investing too much into it, just incase.[/quote]
Nationality and citizenship are the same thing (I used to work in the relevant gov dept) and on the info the OP has given, the two children are British already.
Do you mean residence/settlement?