So sorry for your loss, and for the children to lose their father so suddenly. How awful for you all.
There is lots of poor information on this thread. Firstly, as someone who worked in the relevant government dept said above, his daughters will be able to get British passports. That means they can live here whenever they want, as long as they have somewhere to stay - no need for financial sponsors, and they will have the same eligibility to education and anything else as any other British citizen.
If they do want to come while your nieces are children, your sister-In-law could apply as their parent: www.gov.uk/uk-family-visa/parent - although, the difficult bit is that they would have to come first (ie be living in the UK when she applies). She would have the right to work, but not to claim benefits, or claim benefits on their behalf, and would have to take a test to prove a certain level of English. I'm not sure how she would need to prove the ability to financially support the family, but that is something you could research. The 'easiest' (if the wider family has the funds) would be for her to have savings in her name for a minimum of six months - and it would be a lot, £60,000+, and possibly more if she is also applying for her oldest daughter, on top of housing.
Getting a family visa for your step-niece might be tricky, unless they move before she turns 18 (and as I said, this would increase what your SIL would have to earn/ have in savings in order to apply). This is from the Gov.uk website:
Applying with other children
You can add other children to your application as dependants if one of the following applies:
- they are under 18 on the date you apply
- they were under 18 when they were first granted leave on a family visa and do not live an independent life
This must all be so new and terrible for you now, but you (they) do have options if they want, even if it will be difficult and expensive to come here. When you are ready, I would advise contacting a community law centre or a charity like JCWI, who offer some legal advice: www.jcwi.org.uk/Pages/Category/legal-advice
Good luck, and all the best.