Have been lurking on your thread, and am v v glad you are moving out. Good for you for taking action to change the terrible situation.
Not long to go! The flat - your own space - will be great. It will be the best thing for your marriage and DC.
Your M's living arrangements and wellbeing are up to her. You have hard work with two small children, a lovely, long-suffering DH deserving of being a higher priority, tight finances (but luckily good savings), your job etc. You have already devoted more than enough time and energy to your M!
Btw managing without a bath / garden etc is fine, DC quickly get used to showers, mine did!
Her emails to your DB are shocking, really cold, and as others have said using money to manipulate and treat you differently
. Shame on your DB if he allows her to favour him financially. But not for you to worry about right now!
I suspect that your M might have some kind of accident or incident (a fall, faint, worrying health symptoms) soon, and/or stays in the big house but seeks to make big demands on you. Whatever she does, it is not your fault.
It'd be worth discussing with your DH what (your mother willing of course! She may just return to the UK, hurray!) you will/won't do for her after you move out, if she chooses to remain in the big house. Is she eligible for Spanish healthcare? If so, you could perhaps inform her GP or any health-visitor equivalents of your concerns about her mental state (if you are concerned about this).
Given her past treatment of you and your brother and overt focus on DS (but not DD) it'd probably be sensible to not allow unsupervised access to the DC. Whether she sees them is up to her.
If you want to continue to help her out (you're a more generous woman than many!), try to set boundaries, eg ensure you have time for your own domestics/family/relationship before attending to your M. Perhaps your DH or others could do practical stuff like teach her to online shop? Please don't move out but still be under her thumb.
I feel sorry for your M for her physical pain: my DM has had severe degenerative spinal and joint problems since her early 40s, with horrible pain all over her body, which in my DM's case isn't treatable or relievable, and it is difficult for her to live with. No doubt your M does struggle with pain. But that is not an excuse for her behaviour, or mean that she cannot live alone (when she has plenty cash to pay for help) or that you must care for her. My DM would not expect that, and indeed (though feel bad admitting it) am not sure if, should my DF die, I could have her living with me long-term without my marriage or own health breaking down, and she is much, much nicer to me and DH than your M has been to you!
Hang in there, you are doing the right thing.