There are clearly a number of complexities involved here and it is usually important to separate the legal estate/ownership (the land/property) from the beneficial one. The land register deals primarily with the former so the beneficiary's interest may not always come into play when dealing with the sale/rights etc.
He will still have an interest but it is largely separate from the registration aspects and I suspect this is what your solicitor will explain.
Taking the sale on it's own if there were 3 registered (legal) owners and one has died then you have 2 surviving owners who can then deal with the title. The deceased's executors are not involved in the legal aspects of the sale.
Pookamoo rightly refers to the fact that how they held the title can impact but the tenants in common aspect will invariably only click in when you are down to just a sole legal owner and in this case we still have two
So the buyer will only need to deal with the surviving brothers and any purchase monies would be paid to the solicitor and the two brothers would then take receipt. After that the beneficial estate clicks in and your DH would have a claim on his/the deceased's share of those monies. That is how the beneficial estate is quite separate from the legal estate.
With regards the right of access you refer to if you are referring to a legal easement, namely a right of access which is being written into the Transfer being used to sell the land, then it runs with the land and are not personal to the 2 brothers + DH.
If there is to be a legal easement whereby land owned by the 2 brothers + DH needs the benefit then this needs to be made clear in the Transfer. If DH owns benefiting land then he should be a party to that Transfer
However you mention a licence so I suspect this is quite separate from a legal easement and is simply a deed giving the brothers a right of access so it is personal to them and ends when they die. It would not attach to any land/property they own but simply give them a right of access as and when they wanted it. If your DH also wants that licence then he is no different to anyone else and could presumably be included.
So on the face of it the actual sale by the two surviving brothers is being handled quite correctly.
The wider issue re the right of access seems to be the one on which you need some greater clarity from the solicitor involved