I feel so sorry for your df op, but I also think people should stick to the guidelines, to prevent 10’s of thousands more dying from this Virus. To be honest, my first thought was that of course you should have your friend at Christmas, even if it does mean breaking the rule of 6.
However, on thinking about it, where do you draw the line? If it is okay to have 7, because of the circumstances, then is it also okay to have 8, 9, 10 etc. because of circumstances? Then, in what circumstances is it okay to break the rules? Yes, if someone is terminally ill, but what if it is for what someone else thinks is another valid reason?
Once exceptions to rule are allowed, it is really impossible to decide how, who, when, where the rules are allowed to be broken! People would start to think that, if so and so are allowed to break rules because of x, I can break them because of y.
Looking at your situation, we may all think it is okay because it could be your df’s last Christmas, however it almost certainly isn’t going to be your dsis and dn’s last Christmas? I think this puts another face on the circumstances! You can still have df for Christmas under current rules, you just can’t have df, as well as dsis and dn under the rules. Under the current rules there will be thousands of people having Christmas at home, by themselves or with just their family who live with them, this year (me included). I don’t think the fact that it’s going to be a miserable Christmas because I won’t get to see my grown up children and 4 small grandchildren, is enough of a reason to ignore the rules, which are put in place to save lives!
Ultimately my view in your situation is that having dsis and dn, so that they aren’t alone isn’t a good enough reason to break the rules! Therefore the ball is back in your court, who do you want to have most at Christmas?