Introducing Kittens to Resident Cat?

I have a old cat, Sparkle, who is approximately 13/14 years old. She was a rescue cat. She has been the only cat with my husband and me for ten years. She is a semi-affectionate thing, but not a lap cat. 6 weeks ago we decided to adopt two sibling rescue kittens (Tigger (male) and Cookie (female)) that were found under a bush in the wild at 6 weeks old. We took them at 12 weeks and they have been with us now for approximately 6 weeks.

We have kept them separate in the spare room with bed, food, litter box, etc. and have slowly socialized them. They are both now very cuddly, but still a little skittish if you make a sudden move.

After 4 weeks of keeping them separate we started to gradually introduce them and Sparkle did hiss a little at first and would run away. Then gradually she would just sit in a corner and watch them play and would just growl if they came too close to her. In the past few days Sparkle has become very grumpy and seems to have gone several steps backwards, she is growling all the time now and hissing at me too, even when I try to make time for her and give her treats.

I'm really worried that this isn't going to work. The kittens have been locked up at night in the spare room all this time, but for the past week we started letting them out at 5/6am as they started meowing so loudly! Now they all have free roam of the house all night. Sparkle sleeps on our bed all day and for the most part they leave her alone, usually.

I know some may suggest putting them back in the spare room for a bit, but I can't bear their "crying" at night and also my parents are coming to stay and I need the spare room back as a bedroom. I think I need to find somewhere that my old cat feels safe in the house, somewhere the new kitties can't get to, but not sure how to do this. Can anyone shed any light on this or say if they've had a similar experience? Also, why are my new kittens attacking my house plants? Thanks for reading.

By Emma H