Five weeks is still a very short period of time for her to get used to them, and she's had a lot of change, the loss of her cat companion and then the arrival of these two.
I had one cat that I thought would be fine with kittens as he was okay with other cats as long as he was top dog (or cat!) I had taken in a stray feral that had kittens and the first time he saw them he bolted and then became stressed and started going for the other cats.
I can only imagine that they see the intruders and think that their mother will protect them against intruders and they are in trouble!
Lots of pet remedy and feliway was used and we used to try to make meetings positive, so feeding together, offering treats and best of all is playing together if you can, things like the flying frenzy that they can all join in with.
I do think that sometimes it just takes time, and they won't be kittens forever. I'd try to make sure she has somewhere she can retreat to indside if she wants to, as I wouldn't want her to be pushed outdoors and possibly leaving home; but I don't know if I'd continue to keep them permanently separated as they do need to get used to each other. Once she realises she can put them in their place she may settle,
Only time will tell how things will work out, the one positive is that your cat has lived with other cats before so it isn't impossible, but it might take a while to get to that point.
I know it's difficult, but it's important that you stay chilled and relaxed and acting normally when they are all together, it can be easy to get stressed and hover over them when you anticipate trouble, but it is amazing how quickly they pick up on this and it just exacerbates things.
I'd continue with kwwping them together and see how things pan out.