Your "brother's sister in law" ? Mmmhhhhhhhh. Don't know how good your information is, or where it came from, and why? This is very unusual.
The first thing I'd strongly advise you to do is to try to firm up your suspicions before doing anything which could impact adversely on this woman's life if it turns out to be a load of codswallop.
You say "I am aware that..........is driving illegally on what is probably a dodgy licence." You go on to use the expression "as far as anyone is aware."
So, 1) How are you "aware" that she is doing anything illegal when you continue with "as far as anyone is aware." ? You seem to contradict yourself.
2) Is there anyone within this family circle who can verify your suspicions?
3) At least try to find out, say, if her car's tax disc is expired. Easy to do by simply looking at it which will show the run out date. If expired, there is a fair chance of no insurance. Nowadays, Police Forces have access to databases which hold information on untaxed vehicles. A quick check of the registration number by them could answer that one.
Seems like a glorified family domestic at the root of all this, though?