So upset by my domineering brother

I don't mean to make light of your situation, I can imagine how hurt you feel. ..but could you ask your brother if he'd like a surrogate daughter. My daughter has worked hard all her life, to make the success she has of her life, with no help from her absent father ..I would be absolutely over the moon if she had an uncle who could help her a little financially after all the hard work she's put in to get where she is by herself..

Accentuate the positive. Eliminate the negative.

We are going out for a meal in the evening as they admit they left it too late to to book anything earlier in the day

Wish I had a brother like that, who would have helped my children, when their father and I divorced. I know money isn't everything, but it is something we all need to survive.
Your brother has taken on a father role, I suppose, but they are adults and he shouldn't be interfering in their lives.
There must be an easy solution to this....I know when I was in the middle of the divorce, all, except two of my brothers, kept well away. And may I add one of them was very wealthy.
They are lucky young men to have an Uncle giving them large deposits for their houses. I hope they appreciate it.
They are taking you out tomorrow evening.....you are their Mother and they love you.

There’s no other mothers in law/mothers/grandmothers to fit in. We’re a very small family and I suppose since my mum died I’m the matriarch ?of the family.

Surely you can share the love and pride with your brother? I’d be pleased if my brother was in a position and willing to help my kids out financially. His generosity doesn’t mean your sons love you less, they have an uncle who takes an interest and helps them out. Win-win in my book.

Thanks for your reply MissAdventure?. I suppose what I should have said is that I found out about the house deposit gift on Tuesday this week and told my junior son how I felt as I’ve always tried to tell them gently that they shouldn’t feel that they have to accommodate my brothers life choices when they say that ‘but we’ve got to see **’ . So tonight I feel doubly upset as DS2 knows how upset I was but is still going ahead with seeing my brother tomorrow.

But it’s not a competition

Apologies I I've misunderstood but my initial response to your OP was to wonder about unresolved sibling rivalry between you and your brother. Do your sons feel 'dominated'. I suspect I'd be pleased if I had a brother who was willing and able to give my adult children the kind of financial help your sons are getting. Having lunch with your brother on Mothers Day doesn't seem cruel to me, especially as your sons are taking you out for a meal in the evening. I expect many other mothers will be fitted in between various mothers in law. One of my grandchildren has 3 grannies, all of whom will no doubt get a visit. I'll be fitted in and be pleased to see them.

Well, its not just that your brother dominates: its also that your sons allow it because it suits them financially.
They're all adults, so while I can understand that you're hurt, its up to each of them how they choose to spend their time (and your brothers money)