Make your older cat plenty of spaces where he can avoid the kittens - for food, the litter tray, and sleep. Try to keep them out of his favourite spaces. Generally older cats accept kittens, especially as two kittens will usually play with each other. It's a pity he won't cuff them, as their mother would have.

Also in a few days the kittens need to go to the vets for general exam and first vaccinations. The vet can confirm their genders, and discuss when they can be neutered. Your neighbour could do with neutering her cats as well. But whilst anatomy is OK for sexing them, behaviour isn't.

 

Yes, Tchaikovsky has found his own spaces (we have plenty of outside space available) and we moved his food and personal bowl into my office, where he likes to spend a lot of time unmolested. ISTM that time is the best solution. Not to force anything but to let each side get used to the other.
I was surprised that he took such a timid line with the kittens who are so very much smaller than he is. I guess their odour is different and that is what spooks him, more than the size.
The neighbours have been unable to catch "mum". She is feral and knows how to avoid people.

 

"mum" will need trapping. Hopefully there is a TNR group where you live that can lend a trap and deal with her once she's caught.

And you are quite right about it taking time.

 

Hopefully there is a TNR group where you live
No, there is nothing. We are in the spanish countryside. The nearest thing to TNR is a bunch of drunk weekend hunters who shoot at anything that moves. Mostly their own dogs.
Whenever I see "mum" I give her a couple of cat-treats which she snatches out of my hand and runs off with. In time we might gain her trust enough to get her into a carrier.

 

It is kind that you have taken on these two kittens, and you're trying to gain Mum's confidence.

You're right, I think it will just take time for Tchaikovsky to warm up to them. That he tolerates them okay so soon is a good sign, even though they probably annoy him! It's also helpful that you have two kittens to entertain each other.
You're right not to force the issue for them to integrate; let them take their time.
In the meantime, give Tchaikovsky extra attention and treats, and make sure he still thinks he's No 1. You could also feed him high-value treats in the kitten's presence, so he associates positive reward with them.
Does he have high places he can go to get away from them?

Isn't there a national charity or something similar that will come to more rural areas? It will take a long time to gain Mum's trust, and if you try to grab her, you could end up injured, and any bond you've taken the time to build will be lost.

Do you have any pictures of your cats? We love to see them on here.

 

feed him high-value treats in the kitten's presence, so he associates positive reward with them
That's a great idea, thanks. Some tins of tuna are in order
There are a couple of refuges, but they only take stray dogs (of which there are far too many around here).
As for photos, here are the kittens this afternoon. Basking in the 36° andalucian shade on the fenced in patio. Pepino is the one at top of the photo.

 

Oh, they are adorable!! 💖