Siamese cats often have dominant personalities, so your older cat is likely just establishing her dominance. Since the kittens are so young, this may be a good thing as they can establish their roles in the household and know that she is the queen bee. Make sure she gets lots of attention from you. Siamese cats also get very jealous and need to know that they have your attention. My first Siamese hated when I brought home his six-month-old tabby "brother." They had much the same interaction. I lavished a lot of attention on the Siamese to let him know that he was still king. They eventually got so close that they slept curled up together. When the brother passed away of cancer, my Siamese mourned his loss and would walk around looking for him. Make sure your established cat feels no threat from the younger ones in regards to your attention. Give her treats and pets and whatever she likes. Once she knows she is safe, she won't feel that she has to compete.