Simple, get brother and sister to marry each other!
In all seriousness though, I'm pretty sure only a British citizen by birth can pass on the citizenship to their children. I.e. my daughter is British by descent, born in hk, so she would not pass her citizenship to any children she may have, unless they're born in the UK.
Also my wife (hk) cannot simply move to the uk with me.. she'd have to apply for an expensive visa, an expensive visa renewal, take a stupid test, prove all sorts of financial things, etc. Etc. Plenty of hoops to jump through.
Meanwhile the whole of the EU gets a free pass. Not that I am making a political statement here 