As PP said, you really do need to read the will to see exactly what it says. It would also be best to speak to a solicitor.
It is quite commonly written so that you (I presume you and the two sons are the Trustees?) can use some of the Trust money to help the survivor buy a new home - if you choose to do so (but you don't have to if you don't want) - and then the Trust will own a proportion of the new property.
But that is not always the case, so do read the will.
Whatever happens though, she cannot touch the money in the Trust.
It's probably easiest to explain with a couple of examples. Let's say that the house is worth £200k so the wife's 50% share is £100k and your dad's share which is now in trust is also £100k (round numbers to keep things simple).
Whatever happens she cannot touch your dad's £100k that is in trust.
Suppose she wants to downsize to a property worth £100k. This would release £100k as cash.
Now you (the Trustees) may choose to go along with this and agree for the Trust to purchase a 50% share of the new house.
This will mean that both the wife and the Trust each own 50% of the new house and also each has £50k in cash as well.
Or, as Trustees, you may say to the wife that you do not want to contribute at all to the purchase of the new house. In this case, the wife will own the new house 100% but have no cash and the Trust will have £100k in cash but no share of the new house.
Or you could do any percentage that you like. Maybe you only want to put in 25%?
In that case the wife would own 75% of the new house and have £25k in cash and the Trust would own 25% of the new house and have £75k in cash.
It's totally up to you as Trustees as to what you do.
So, let's say that you have decided to go in 50/50 with the wife. So the Trust now has a 50% share of the new house and £50k cash.
Great, we can just divvy up the cash between the three children, right?
No!
That money is still an asset of the Trust - you don't get your hands on it until the wife passes away or if there is some other condition in the will that is met (eg sometimes remarriage or even selling the home).
Up until then it is normally written that the wife is entitled to any income from the Trust but, of course, she can't touch the capital.
So, with the £50k cash you now have in the Trust it would probably be best to invest it, whether this is in unit trusts. investment trusts, individual shares, government bonds etc.
I suggest that you should speak to a qualified advisor about this.
Any capital growth is retained by the Trust, but any income (such as interest or dividends) is paid to the wife.
For example, you put the £50k in a savings account. According to Moneysavingexpert, the top rates at the moment are around 5%. So £50k in a savings account could earn £2,500 per year in interest and that must all be paid to the wife.
That's great for the wife but not so great for the children.
Alternatively you could invest in things that go for capital growth and don't provide any dividends at all. Great for the children, not so great for the wife.
However, as Trustees, you are required to take the interests of both parties into account (unless the will says otherwise) when deciding what to invest in.
This would normally be done by selecting a mix of investments, some of which concentrate on capital growth (good for the children) and some which concentrate on providing income (good for the wife).
I really would recommend that you speak to a qualified advisor about this.
Then when the wife finally passes away the assets of the Trust can then be passed out according to the will.