Derrick Bird's twin brother was given £25,000 by their father before the old man's death, his will revealed today.
Neither the killer nor his other brother Brian appear to have received anything and the rest of their father's wealth passed to their mother, Mary Bird, who is now 90 and very frail.
A legal expert said Mrs Bird would have had to decide whether to include clauses in her own will cutting the bequest she gave Bird's brother to allow for the money he had already received.
The disclosure added weight to the theory that the taxi driver launched his gun rampage in Cumbria after rowing with his family about his inheritance.
Bird's father Joe gave the money to his son David - believed to be the first victim of the massacre - before he died in October 1998 at the age of 82.
His will, which is a publicly available document, included a special clause stating that David would inherit £25,000 less because of the cash he had already received.
But this was never applied because his entire estate went to his widow.
There has been speculation that divorced Bird, 52, had money problems and snapped after an argument with his twin brother over their elderly mother's will.
He appears to have been much less well off than his siblings, who both lived in Lamplugh.
David lived in a large farmhouse and made money when he sold off some of his land to a developer who built four modern houses on it, while Brian lives in a big period detached house.
By contrast Bird lived in nearby Rowrah in a scruffy pebble-dashed mid-terrace house.
Mrs Bird had her own home in Ennerdale but had not been seen there since she attended the local primary school's Christmas play.
Some neighbours suggested that she had been living with Bird for several months before this week's killings.
Mr Bird drew up his will in November 1987, leaving everything to his wife unless she died before him, in which case his estate would be split equally between his three sons.
Then a year before his death he added a codicil.
This stated: "Having transferred money to my son David Bird absolutely, I direct that my son, the said David Bird, shall bring into hotchpotch upon the division of my residuary estate the sum of £25,000."
This "hotchpotch" clause meant that David would inherit £25,000 less than his brothers when Mr Bird's estate was divided up.
However, probate expert Elizabeth Young, a partner with Brethertons solicitors, said: "If everything went to the mum, those hotchpotch clauses had no effect.
"But there may well be hotchpotch clauses in her will - it's possible that mum also has a will in similar terms. That would be quite common for a couple.
"It is a private matter for her, but it may be she had a copy of her will at home that somebody had seen or knew about."
The total wealth in Mr Bird's sole name at his death was less than £10,000, although this excluded property and bank accounts he shared with his wife.
Intriguingly, Mr Bird also included a clause in the codicil stating that any other gifts worth more than £500 he made to his sons before his death should also be subtracted from their final inheritance.
But there is no evidence that Bird received any money from him before he died on October 31 1998.Reuse content