This 100 per cent!
OP most of us are brought up to be kind and considerate to our families. But as you become an adult and more independent you also have to be kind and considerate to yourself! Times are changing and there is less and less state help now if you are too sick to work, need to retire, need an operation with a LONG waiting list, a suddenly unemployed, want to retrain to improve your career.
I think the pressure on you here is that you are afraid that A and B will pile on you and call you mean.... "Oh the Horror @CFornot123 is MEAN!!! She won't give me £2 grand for my hobby! Poor Me. Woe is me! Everybody hate CFornot. Boo Double Hoo" "C Fornot you are such a selfish child. I am asking you to give the money because its a way of me showing B how much I love them and how much I am willing to pressurise others to give B money. So B will be even more grateful and loving to ME."
Im glad B only wants money and doesn't need one of your kidneys.
I've been there and I guarantee this is what is going on.
Just think. They say this. So what. How long can they keep it up?
Is it worse to hear them complaing? I bet they complain a lot. I bet life is hard all the time for B and B needs special consideration. Or is it worse knowing that you have been royally CF'ed and will continue to be CF'ed at every opportunity by both.
Sieze your independence. I'm not saying go NC. Just stand up for yourself. Yes they will keep asking and bad mouth you but so effing what? Let them!! Don't they already do that anyway? What's new? they can't keep it up forever. In fact you can come back with the same nonsense and say things to them like "You only care about me for the money." See how they like that. (Actually that would be a last resort. but the concept of throwing it back on them is a sound one)
B. Do they work? Oh CFornot - if I don't get this £2k I won't become a professional photographer to royality, compete in Olympic dressage, play at Glastonbury. If £2k is all that's stopping B. Here's an idea. They can WORK and SAVE up. "B... I'm thinking of you. It would be so much more meaningful for you if you did this yourself. etc." You could have a lot of fun with that one.
I note B did'nt ask for a loan. She could get an interest free credit card and pay it back over 24 months at a rate of £83 a month. Ask her what other plans she has to finance the expensive hobby. I bet you are the main plan and will continue to be.
If she can't afford that then she can't afford the hobby anyway and any money you put in will be wasted. Once you make one payment on this hobby B will never stop. And the pressure will be greater because time and your money will already be invested.
There comes a time when you have to think of yourself before your siblings. It is not mean, it is not selfish and it is not evil and I am sorry that your family can't see that. What's the rush anyway. Why doesn't B prove themself in this hobby and you might consider it. They sound too lazy and entitled to do that. We don't have to pander to lazy and entitled people to stop them from calling us mean!! Why must you dash up with handfuls of cash before you've even decided how you are going to spend this on something substantial or keep for life's emergencies. What if you have children later on?
Tell DM to take the £2k out of your inheritance and give it to B. Problem solved.
You don't get many windfalls in life that could get you through difficult times.