I am a bit torn here, because it does seem he was mislead a bit, and thought he was getting more, and is not being unreasonable to be a bit miffed.
On the other hand, it does seem a bit grabby, and like someone said further back in the thread, you have to question your morals if you are more miffed about the (lack of) inheritance than your relative dying.
I have never received a single PENNY in inheritance from anyone, but I have known a few people who HAVE inherited. One woman I know inherited THREE TIMES from various older relatives between 2012 and 2018. A total of £250K. And for some reason she still moans about money, as she only has £53K left. 
She did put £100K on a house in 2016, but that doesn't explain where the other £100K has gone!
When asked, she just responds with 'it's just gone on living expenses!' Fuck knows how she would have survived without the bastard inheritances then! £250K would set me and DH up for life. VERY annoying! 
My mom's cousin's husband was very solvent - had his own business, and a big 7 bedroom house, a villa in Florida, and a proper fancy car. My mom's cousin married him in the 1970s when he was already quite well off. She helped him run the business and they were very happy for many years.
Around half of my extended family; cousins, aunts, second cousins, in-laws etc, sucked up to him and her in the hope of getting something when he (and she) died. They did work for free for him, offered to do his garden, look after his pets, clean his house etc, and all sorts.. My mom's cousin died first, and her husband died 6 months later...
When he died, the whole estate was left to their son, and daughter-in-law, then. Not a penny was left to any of the sycophants. I mean, he never promised anything, but they still expected something!
So the moral of this story is, don't expect fuck-all because no-one is entitled to anything, no matter HOW much you creep up their backside and 'do loads' for them. The money STILL went to his son and daughter in law, who had naff-all to do with creating and running the business.
They inherited and sold it, and made seven figures from it. They are sitting very comfortably now, aged 48 and 49, retired, and never have to work again, whilst the 9 or 10 relatives who crept around my mom's cousin and her husband, don't have a pot to piss in.
I don't know anywhere where you can buy a house for £15K to £20K by the way!!! I mean, there was an offer (some 10 years ago,) from the council, to buy a house in Stoke on Trent, or Hull or I think, for a POUND. But you had to also take a £40K loan with them to do it up, as they were uninhabitable. But now I think they are worth £80K or so.
But even in the cheaper areas in the UK - north east, parts of Wales, and the midlands and the north etc, you'd be paying £80K to £120K for most half decent homes.
Niblings IS actually a real term by the way! A collective term for nieces and nephews. Cute word isn't it? 