Wait, your DP and his siblings haven't received anything but their DC (woman's DGC) did.
What on earth is wrong with that? not usual but money passes to the DGC rather than DC.
I am confused by the nieces having anything to do with it to and anyway they are only getting £2,000 as opposed to £120,000
You seem to be incensed that your DP and 2 DC (step and other) haven't had this £120,000 split but the will is doing what the DGM wanted that is to bypass her DS (and other DC) and why should she leave money to DGC eg a step DGS?!
It does not matter if your DC who is the step son inherits nothing from his DF - or his step DGM that is her wishes and who knows his DF position may change in the future even if he does rent.
personally in light of this I think both your assets should go to both sons equally but your DP doesn't have to treat the step son equally at all.
You could of course in this situation favour your eldest son more than the youngest but this could be seen as unfair.
£120,000 now may not buy much if you're thinking in terms of property and even more so in the future!
You could look to paying off mortgage sooner if you can and THEN helping eldest DS with property when he comes of age and of course not helping or not so much youngest DS in the future. This is especially fair as you think DF of eldest DS won't presumably have a lot, but what about his DP (eldest sons DGP) - they may choose to leave money to eldest DS or eldest DGS.
There was a similar situation a few years back with friends... DGM died and left estate to her 2 DGC and DS. Nothing was left to another DS. DGC as adults felt sorry for the other DS so gave him some money (I don't know whether he had DC). This was fair in that instance.
In your instance your DP sounds totally selfish and was expecting all sorts. Sounds as if his DM had the measure of him hence leaving him nothing!