I also think that your best bet would possibly be action against the financial/pension company, on the grounds that there were no safeguards in place to prevent your mother doing exactly this - basically, they gave YOUR money straight to someone else, while you were a minor, and had NO system of checks to make sure that it wasn't stolen by them.
Lots of good advice above on how to start off getting all the information you would need.
However, one thing I would add is - you also need to sit down and work through exactly what you feel you would be prepared to do about this with regard to your mother. Others are saying - take her to the cleaners, do whatever it takes to get your money back, etc. Now I agree completely - she is scum and has basically stolen a large part of you and your sister's futures, and the futures of your families. I would be cutting contact without a second thought. But if you KNOW that that isn't what you want or could cope with, at some point you are going to hit a wall - in your actions either against your mother or against the company. She is going to come to you whining and crying that she is going to be ruined/end up with nothing/all she did was try and take care of you/thought she acted for the best etc.- when you either a. try and put a charge on the house or b. get her into trouble with the pension company/police.
Here is my advice if you feel you will fall at this hurdle.
You need to think carefully here about the future. Your mother is a dead weight around the necks of her family, a ruiner, a loose cannon. If you let her get away with this, financially, you know what will happen? She will liquidate what money she has, run through it, and then expect to be bailed out by you for the rest of her life. She will bleed you dry, ruin your own home life and possibly marriage, and it still will never be enough.
So you need to tell yourself that by taking this action, you are doing what little you can here to prevent that. If you can secure money from her or the pension company, not only can you perhaps undo this injustice to you, your dad, sis and any future children, but you can at least STOP your mother from pulling you ALL down any further. If you can end up with control of what little money she has left, you can - without telling her - mentally put some of it aside to make her an allowance in her old age, or to fund care for her. If you don't, she will waste and waste and will STILL end up destitute - the only difference will be that you won't have that nest egg there to possibly help her.
Maybe that line of thought will make a difference to you when you need to be strong - because you will need to be. She will pressure you every way she can, I am sure, because she sounds thoroughly evil. I sincerely hope you will put her out of your lives once you have justice on this.