I would broadly agree with those who say that your mum took into account any possibly 'inequality' when she made her will, and that is that.
Having said that, I think most of us in a similar position would feel a bit uncomfortable. I am interested in your husband's position on this and wonder how you divide up money in your household? For instance, when we inherited our parents' money, my sister felt quite strongly that it was 'hers' - not her husband's. She keeps it for treats, for helping out family etc. (FYI they inherited nothing from her husband's family)
We however, have always pooled our money, and so decisions about 'helping out' family members has always been a joint one - and indeed did cause a little friction awhile ago.
So in your position, my sister would make the decision completely for herself, whereas I have to make it jointly with DH (and so would he)
I would say that if you decide to 'help out' make it one-off, so you know where you stand both emotionally and financially. Set aside a similar amount should the other brother come calling. Certainly don't make it any more than you can reasonable afford. If you had a baby young, you must have had some tough financial times, so don't make yourself short now - I'm sure your mother wouldn't wish it!