She may not have the money any more - that is true.
But Op has said the MIL does have the money, or at least did when her DH helped with his fathers estate!
Op, I feel for you both and don’t think you are entitled and grabby. I have been in exact position, except with larger amounts.
Apart from paying for my wedding (but also DBs, before mine, as his wife was from abroad) I never had a penny off DPs, even when buying my first house at a young age. Not one penny. Worked hard, bought houses alone then with DH.
DB has worked 2 years of the 43 he has been out of school. He openly admits that he thinks the world owes him a living - or rather the Govt and parents, who gave him monthly money all of their lives, despite mum denying it to me.
When DB’s wife managed to secure a mortgage to buy their rented house, from her wealthy employers, they asked mum for the deposit, in region of 50k. All the family knew, cousins etc except me. (Our dad had been dead quite a few years then, not sure he would have given them it)
I found out by accident about 4yrs later when mum was ill and I was managing finances (I live closer, but still 2 hrs away, and obvs helped more even though DB didn’t work ☹️).
I was devastated - it wasn’t about money at all, as you say, it just felt that DB was favoured over me, definitely by mum. Plus the secrecy - my cousins felt awful knowing that I didn’t know, but didn’t feel able to tell me.
I went NC for a short time, but by then mum had been diagnosed with terminal cancer and I couldn’t just walk away. I knew I couldn’t live with myself after losing her if I did.
I did get offered the deposit money in her last months, and decided to accept it, partly to pass to my DC and partly to stop my brother from gloating when she died. But I reckon he probably had at least another 20k off our parents.
I have since been told by a cousin that he wrote to her just after I found out about the deposit, insisting he deserved everything and that he would take her to change her will when he next visited!
We did PoA not long before she died, so I knew the solicitor had her original will at that point, but I did wonder if DB would produce a new will, right up until we walked into the solicitors office. As it was we (DH and I) did everything regarding her house and estate. DB was a joint executor but true to form, did nothing.
We have 2 young adults and aim to treat them as equally as possible, although one is currently at uni, youngest may not go, so there is potentially a discrepancy there. We are feeding money into their ISAs to use for house deposits, so in theory they will have the same from us, regardless of when they buy.
At least you have been made aware of the money given to your BIL, so your DH can make his own decision about continuing to support his mum in the future. I wish I had known earlier but think my mum was threatened by DB not to tell us, as he knew that if she gave me the same it would reduce his eventual inheritance. Pure greed.