I read that Lady of Hay book (and hated it, far too many excuses for modern day rape and violence to women, with the lead female seeming to enjoy both as a sign of love) but was very interested to learn more about Matilda de Braose because of it. Unfortunately there doesn't seem to be that much known about her, even the place she died is disputed.
She was also known as Maud de Braose and she lived from 1155 - 1210.
She and her son were starved to death in Corfe Castle (or possibly Windsor Castle) after Matilda and her husband fell from the favour of the King.
The reason given her husband owed the king a large amount of money, and the kind demanded their eldest son as his hostage until the money was paid.
Matilda refused to send her child to a king who she claimed murdered his own nephew so the king seized their property and put Matilda and her son William in prison in the castle (Corfe or Windsor, nobody seems certain), where they starved to death together. Her husband escaped to France but died there not long afterwards.
The deaths of Matilda and William shocked people so much that a clause was added to the Magna Carta when it was signed in 1215 because of them, stating that nobody should be prosecuted, imprisoned or outlawed without trial.