Why does my adult cat groom new kitten then hiss and swat at her?

It is OK behavior but not great. I am worried that this could escalate into a serious fight. I suggest you try using some Feliway diffusers in the house and do some environmental enrichment for them. Those two things should help. ( https://www.feliway.com/us/Products/FELIWAY-MultiCat-Diffuser ) ( thedrakecenter.com/materials/environmental-enrichment-indoor-cats ) ( https://www.youtube.com/watch?v=AejmrLa_Cs4 )

The Step by Step Process Put other cats away before you bring the new cat home, so that you can take the cat to its room without encountering the other cat(s). Then close the door securely and go visit your other cats. They will smell the new cat on you. Give them treats to positively reinforce this experience. A few days later, in order to further acclimate the cats with each other’s scent, it is good to exchange the new cat's bedding with that of the resident cat. The next step is to rotate rooms daily for at least 2-3 days. This allows the new cat explore the rest of the house while the resident cat spends some time in the new cat's room. This will give your resident cats a chance to smell the new cat and rub their own scent on objects. The cats should be fed in each area with wet food twice daily, and it is also good to play with the cats in each area at least twice daily. The practice of setting up a schedule of feeding treats of wet food, as well as a playtime routine, minimizes anxiety and makes the cats feel more secure. Keep the cats in separate areas for as long as it takes to allow them the chance to become desensitized to the smells and sounds of the cats in the other area. When they all seem particularly relaxed with this process, begin to expose them to the sight of each other. A good way to accomplish this visual introduction is to bring the new cat(s) in a carrier to meet your cats and sniff each other through the carrier wire door. Place the carrier on the floor and allow them to meet this way several times a day for about an hour. Continue these meetings for several days or until they remain calm in each other's presence. If and when the cats are able to stay calm in each other's presence during these meetings, it is time to let the new cat out into the rest of the house for a few minutes. The length of the visits can be increased gradually each day. Depending on the personalities of the cats involved, this process may take a few days or a few months. If it appears that any fighting maybe be close to occurring while you are supervising, put the newcomer back in his/her room and proceed more slowly. If those supervised play times go well, you can begin to allow them to cohabitate in your home. If any aggressive behavior occurs at any time, begin the introduction process again. It is very important to understand that this process can take as little as a few days, to sometimes much longer. It is all dependent on the cats involved. If things don’t go well right away... One of the most important steps to dealing aggressive behavior between cats is to recognize the different types of aggression and understand their underlying causes. A short list and description of some commonly seen aggressive behaviors include: chasing; ambushing; hissing; swatting; and/or biting. If you see these behaviors, seek help from your veterinarian. Most cats get along fine if the introductory process is done correctly. Please seek help from your veterinarian if you feel that things are not progressing well, or if you have other questions.

IT can be common for two cats to get along sometimes and not other just like children. The cats are working together to figure out perfect harmony in there environment. I would not be concerned unless you notice abnormal urination/defecation or you feel like the two cats are constantly fighting. IF they are having occasional disagreements that can just be normal pet behavior. If you feel the younger cat has a lot of energy I would recommend trying to increase play by hiding toys and treats around the house. Hope that helps

Most cats will settle down within a few weeks. Kittens do play rough and it can be hard to tell. If they willingly come together, play willingly and you don't see any bullying behavior (avoidance, hiding, arching the back and puffing up the tail) they are likely playing rough. If they play too rough they could injure each other by accident so continue to separate them when not supervised but most kittens will not require this. Giving each kitten one on one play time with you to burn energy, feeding out of a food puzzle and providing cats trees/other environmental stimulation can also help. Continue the current therapy as well. I hope this helps.

It may take time for Lilly to accept your other cat even though she is the smaller new cat. Some cats just don't do well together so if this behavior continues or gets worse I would recommend either talking to a veterinary behaviorist (if there is one in your area) or considering finding someone who can provide Lilly a great life as an only cat. Thank you for using Petco Pet Education Center, formerly Petcoach.

This is very normal behavior. Make sure you provide plenty of litter boxes and food and water bowls to decrease the stress and competition of the kittens. Also, you can put some feliway diffusers in their room to help calm them. Feliway is a cat pheromone to help calm them but it doesn’t work on everyone. Give them some time and hopefully they will start to warm up to each other. I hope this helps!

Thanks for using PetCaoch! It’s not uncommon to hear about behavior like Alyssa. It can take a lot of time and patience for them to get along but here are some tips for decreasing stress for Alyssa in the short term. -Feliway- calming cat pheromone- comes in many forms from pet stores or online. 

 - Adding a litter box. Studies show that the ideal number of litter boxes for a home is #of cats +1 . This is regardless of how clean you keep his current litter box. 

- making sure she has plenty of “high space”. The safest (and therefore least anxiety-inducing) place for a cat is up. If you don’t already, get a cat tree or put up a shelf or two that she can go perch on. 
 Feeding them in separate rooms with two or three water bowls throughout the home is also a good idea as perceived competition for resources is a common problem in cats. Best of luck to you and your kitties!