Your 15 year old MaMa Kitty will probably not like the new additions! For one thing they will be much more active and will disrupt her normal sleep cycle. Further, she will be territorial about her surroundings.
Some cats will develop inappropriate behavior (such as not making it to the litter box) when a new cat is introduced to the environment. You can alleviate much of MaMa Kitty’s anxiety by spending extra time with her to assure her she is still number one in your heart! Keep the kittens isolated initially and introduce them slowly to MaMa Kitty. Pay attention to prevent the kittens from harassing MaMa Kitty.
Also don’t expect MaMa Kitty to share her food bowl or litter box with the new kittens. For one thing she needs senior cat food and the kittens need food for growth. There should be at least two litter boxes available.