@Onionpeeler
I'm in agreement with most people here, OP. This is not your responsibility, but the children's parent to solve. I'm astonished he has accepted funds from the children's grandparents to the extent he seems to have and, now they are running low, turning to you.
If you lost your job, would any of the family be able to come to your financial rescue? No? Then that's where any "spare" funds should go - into an emergency fund if your work/health circumstances go downhill, as a buffer against how the economy is likely to suffer for the next few years post-Covid and Brexit and also towards your pension - which is also likely to suffer if, for example, the triple lock is removed.
Also, given how stretched the NHS is, what if you needed to pay for private medical care? I have a family member going through that right now and I myself have had to self fund for treatment which would have been delayed on the NHS. All things to consider when you are being asked to pay for your niece's education and then, probably her sibling..
Your brother needs to see where HE can organise his finances more effectively, I think, maybe taking on extra paid work, if possible.
I see a couple of people have suggested you occasionally doing a supermarket voucher/online food top-up for your niece and I think that would be a good idea if you are able to afford that?
Stand firm OP. No need for guilt or obligation on your part. 🌹