I agree with the "give as needed" mindset. IMO, it's ok to give the others the same amount - if you (general) can afford it - but as PPs have said, what if they really need it later? If they don't, towards the end of life, you can always "make it up" to them or in your will. Personally, if I felt a need to make it up, I would do it while still living, if I could, and let my will divide the rest equally, but that's just me. IMO, so much depends on what parents can afford and still stay solvent themselves. AC need to realize this and not act like babies.
But if two AC find themselves in need and the parents only want to help one, IMO, that's totally lopsided. Fizzers, what you describe sounds very unfair. I am so sorry! I'm not clear, however, on whether your mother just complained while helping you or refused to help you at all. If she still gave you the money, despite her gripes, maybe she did the same with your sister and nieces? Helped them out but complained while doing it?