It is really a case of the lack of existence of any established legal trust, rather than the existence of one, enabled Mrs Ferrars to do as she wished with the family fortune.

Mrs Ferrars appears (and there is very little legal terminology to guide us) to have sole control of the Ferrars fortune. Certainly she acts as if she has no legal fetters on her capricious behaviour.

Her husband must have left her the Ferrars property in its totality, and seems not to have set up any trusts in his will, or , indeed, during his lifetime, for the devolution of that property upon her death.

A sensible method of ensuring the estate eventually evolved to ones heirs and not to any new husband of the widow was for the now deceased husband to set up a trust in a will , made during his lifetime, leaving his property to his heirs, and providing a jointure for his wife to enable her to have sufficient income to survive during her widowhood.

His widow would normally be the guardian of the children if they were underage and would adminsiter the estate till they were able to inherit .

However, it was not entirely unknown for the will to specify, and a trust being set up at the same time, to enable the surviving wife to be given a life interest in the whole property, with the estates eventually devolving on the heirs after her death. This appear to be the position with regard to the Rosings estate and Lady Catherine.

However, Mrs Ferrrars does not seem to be so emcumbered. In fact, her only legal obligation under Mr Ferrars’ will seems to be the paying of annuitiies of 3 old retirered staff:

“Certainly not; but if you observe, people always live for ever when there is any annuity to be paid them; and she is very stout and healthy, and hardly forty. An annuity is a very serious business; it comes over and over every year, and there is no getting rid of it. You are not aware of what you are doing. I have known a great deal of the trouble of annuities; for my mother was clogged with the payment of three to old superannuated servants by my father’s will, and it is amazing how disagreeable she found it. Twice every year, these annuities were to be paid; and then there was the trouble of getting it to them; and then one of them was said to have died, and afterwards it turned out to be no such thing. My mother was quite sick of it. Her income was not her own, she said, with such perpetual claims on it; and it was the more unkind in my father, because, otherwise, the money would have been entirely at my mother’s disposal, without any restriction whatever. It has given me such an abhorrence of annuities, that I am sure I would not pin myself down to the payment of one for all the world.” 
Chapter 2

This is why she can play with the fortunes her sons will eventually inherit.

There appears to be no legal prohibition preventing her from disinheriting Edward and setting up a trust to give Robert a thousand pounds a year then leaving the bulk of the estate to him by her will. She appears to have given Fanny Dashwood, on the occasion of her marriage, £10,000 and this passage in Chapter 50 seems to indicate that that sum would be the most Edward could ever expect to recieve under his mothers will, or as a gift during her lifetime :

In spite of his being allowed once more to live, however, he did not feel the continuance of his existence secure, till he had revealed his present engagement; for the publication of that circumstance, he feared might give a sudden turn to his constitution, and carry him off as rapidly as before. With apprehensive caution therefore it was revealed, and he was listened to with unexpected calmness. Mrs. Ferrars at first reasonably endeavoured to dissuade him from marrying Miss Dashwood, by every argument in her power; — told him, that in Miss Morton he would have a woman of higher rank and larger fortune; — and enforced the assertion, by observing that Miss Morton was the daughter of a nobleman with thirty thousand pounds, while Miss Dashwood was only the daughter of a private gentleman, with no more than three ; but when she found that, though perfectly admitting the truth of her representation, he was by no means inclined to be guided by it, she judged it wisest, from the experience of the past, to submit — and therefore, after such an ungracious delay as she owed to her own dignity, and as served to prevent every suspicion of good-will, she issued her decree of consent to the marriage of Edward and Elinor.

What she would engage to do towards augmenting their income, was next to be considered; and here it plainly appeared, that though Edward was now her only son, he was by no means her eldest; for while Robert was inevitably endowed with a thousand pounds a year, not the smallest objection was made against Edward’s taking orders for the sake of two hundred and fifty at the utmost; nor was anything promised either for the present or in future, beyond the ten thousand pounds, which had been given with Fanny.