I am doing this on behalf of my niece. Her mom lives in SA, which has a husband and they got married after my niece was 21 so he never adopted my niece although he actually brought her up. My niece becam british almost 10 years ago and her husband is british, they have a daughter of 4. My sister is 54 and her husband nearly 70 they want to come to the uk to be close to her daughter. My sister and husband has been visiting the uk a couple of times but the trips and the expense of all that getting too much for them. My niece's husband will sponsor them but because my sisters husband isnt my niece's legal father this cant be done. Does anyone have any ideas to how to go about it. P.S. I am her sister and I became british as well as my son and husband are british. So she has other family in the uk as well.
The one thing that can be said about immigration and settlement rules is that they are complicated. I suggest it would be very worth while spending a little bit of money speaking to a specialist immigration lawyer. (You can find a local one via the Law Society either by phone or online). You will quickly be given the available options in the particular circumstances of your case.