Question about my cats. Can anyone help me?

  Ok, so I have two kittens they are almost 6 months old, and I brought them home to my house where my gramma has two cats of her own. They are one male and one female (both fixed) and they are 10 and 13 years old. My kittens are both females. So, I brought them home and well my kittens they are the ones hissing at my older cats. They have only been living with one another for about two days but still I just wanted some opionions. So, my kittens use to be outdoor cats, so i dont know if that has anything to do with it, but I just wanna know like. I read that cats hiss because they are defending there terriotory, but my house was my older cats terriotry first so why are the kittens hisses at my other cats? what can i do to help them? Is this a sign they won't get along with one another?

Please help me. I just need ressurnce that they are all right and that I haven't done anything wrong and it will work outl.



Thanks so much!
Your kittens are just scared they not used to the new older ones and they need time