They did!!!! She was one of 4, 2 boys and 2 girls. One boy got the family business and the other got the family home and their second home. The girls got nothing, because they didn't 'need' it. The brother who got the property was married to a women with children from a previous relationship (and had no children of their own). All his inheritance has been siphoned off to her children. DM doesn't mind that one brother got the family business as no-one else wanted it, but she is still resentful that 1/2 the estate has gone to her BIL's step children.
I just wanted to pick through a few assumptions here that might be worth looking at.
Firstly, yes, her parents divided up their estate according to perceived need of each child. They decided their daughters didn't need any money presumably because they were already financially more comfortable than their brothers, so by passing their assets to their sons, they evened the odds among their children. Is this accurate? And is this wrong?
Secondly your mother doesn't really seem to resent much about this arrangement, apart from the fact that one of her brothers later chose to pass his assets onto his stepchildren, not having any biological children. Those children might not be biologically his, but he married their mother and presumably formed bonds with them and cared about their wellbeing, which is why he passed his assets on to them.
This seems like a pretty reasonable choice, far more reasonable than, say, cutting them off and giving that money to his nieces and nephews who are blood-related, for example. I am not sure if that's what your mum thought he should do - might be worth asking her exactly why she resents it and what she thought he should have done instead.
I think if instead of taking offense (easier said than done) the better thing to do is to take a step back and genuinely try to understand why your mum wants to do this, while assuming that she has a positive intention.
You say you don't think she's doing this because she doesn't love you, it just feels that way, because money feels like love to you. That's also a perspective worth inspecting, non-judgementally. Why does money feel like love to you, if you don't actually need it? Why doesn't her choice to give more time to your family feel like she's given you more love than the others? Maybe money feels like love to you, but time feels like love to her.