Hi James,
Sad events and I am sorry to hear of them - you are, though, fortunate to have a legacy that cannot be bought, namely having had a father with the foresight to take you and your brother to Le Mans every year from the age of 11, with the good memories that will always remain of those trips. And what a great tribute to go this year.
I'm sorry that I can't help personally, as my MGA only has room for 2 people including the driver. A couple of thoughts that might help the process of making sure that the pair of you get to La Sarthe and back in June:
1) It would help people to know where you are located, when you can and cannot travel, and when you need to be back.
2) I understand your concern that your uncle might not appear to be the most reliable solution, but as the scheme you propose involves what would effectively be a complete stranger to your family taking the pair of you (at age 17 and 14) to a foreign country for up to a week, unless you are going to do a sneaky runner from home and then appear on the front page of the newspapers a week or so later having returned "safe and sound", I suspect that, at some point, a little more formal family approval of the scheme will be needed for the peace of mind of both your family and those offering to take you - best involve them early, probably only once you have a string of positive replies to this thread to show them.
The second point is pretty vital - it is meant in the best way, to help sort it properly, so that everyone involved is happy with whatever scheme is concocted and a great and memorable trip ensues. As well as bringing offers of lifts - there are three offers on the thread already - the replies to the thread could also be a very powerful persuader to the family that they need to allow you to go and guarantee your trip, rather than "offer". The replies can also provide some reassurance to them that there are a whole bunch of good-natured people on CA who genuinely want to help for all the right reasons.
Of course next year you will be 18, hopefully driving and owning a car of your own (or borrowing one), and can take off at will! I do wish both of you the very best for getting there this year, and hopefully we'll see you at one of the CA gatherings.
Do keep us up to speed with what is going on via the thread
MG Mark