Absolutely ask for your full share now so this doesn't happen again.
Write out your points before you meet them.
The idea that your feckless brother remains unemployed because working for someone after ownership of a business is unpalatable to him,..........that you should sacrifice your inheritance in these circumstances....... is utterly preposterous.
I mean this with the utmost kindness, but you need to be tougher with yourself and up your expectations of yourself.
Having children can throw up some very unexpected challenges.
You need to remind yourself that you really need to reframe this doormat narrative you have of yourself, as it really will be a massive disservice to both yourself and your future child.
Bad health, job loss, a child with challenges can and does happen in the blink of an eye.
How would you feel handing over this money, knowing it could have been a huge buffer to your new family?
Your brothers choice to fxxk up his marriage is NOTHING to do with you.
Stop allowing your parents to involve you and your family in your brother's decision to cheat on his wife AND his children.
HE has let his children down.
HE cheated on his children.
Do NOT allow your finances to be tangled with his.
Don't beg/implore your parents to understand you, that will serve NO purpose.
Spell out to them that you do not feel in any way obligated to be your brothers keeper.
His life choices are NOT yours to suck up.
If you can't do it for yourself do it for your unborn child.