Hi all you helpful lot!

I currently live in a house with one housemate & my two cats (both female) who are just over 2 and a half years old ( sisters) who i have had since they were kittens. They are mainly house cats due to the house not having a cat flap, however i do open the windows so they are free to roam outside if i'm about in the house that day, however they don't stray far & seem happier curled up inside! (especially now it's winter, & can't say i blame them!)

Anyways my housemate has decided to rehome / longterm foster an elderly cat who is about 15 & suffers from numerous medical problems. (She volunteers for the RSPCA)

What is the best way to introduce them etc?

My housemate has bought a litter tray to have in her room, where the new cats food will be as well - she's not very good with stairs so i think she may well end up living mainly upstairs.

My two girls have a litter tray downstairs in the kitchen. There food / beds are also in the kitchen / dining area.

I'm due to have a baby in January so i'm trying to get them into the routine of being shut in the kitchen area overnight ready for the babies arrival.

I'm really worried about how my cats are going to cope with a new cat, they are not vicious in the slightest though ! I just worry because they spent a few months living at my mum's house where there was 3 cats & a dog & they absolutely hated it, and lived pretty much upstairs completely refusing to go anywhere near the other pets. But i'm sure this will be different as this new cat is elderly & ill.
I just worry because they've always been the only pets in the house living with me in all the time that i've had them so they're used to being on their own.

Any advice welcome!
My housemate is due to pick up the new cat at the weekend.

first off you should ask the re-homing center how well the older cat gets on with other cats because of her age and poor health I suspect she will at first feel more threatened of you're two cats then the other way around. even though the older cat will be kept upstairs you're other two cats will certainly know there is now a newcat in the house. The best thing to do is keep the older cat upstairs with the door closed to allow her to settle in. . We were told to also leave some thing in the room that had the sent of your other cat on to also help the new one settle in.

After a few days introduce the cats but keep a firm hold of your cats so that they can only sniff each other if that seems to go ok then you can take things a bit futher but make sure you can quickly grab your cats should they startfighting. There may be a bit of hissing fromm your cats but that's just natural asthey will be just establishing who's boss. If the first meeting gose ok youcan also try feeding them together.

Me and my family have had cats practically all our lives and not once have we had to send a new cat away cause they can't get on with my other cats. I've recently adopted a new cat and introducing them was sooo easy, although now the new cat Evie wants to play a bit too much which has caused some swiping.

This is how me and my family have done it so feel free to ignore the advice as it may not work with everyone or every cat. After bringing the new cat home, let her wonder around the house, basically leave her to it so she can suss out her surroundings, but also let your other 2 cats wonder aswell...i wouldn't stuff one cat into a room for a few days...i would let them all roam together so they can smell each others scents. But.....keep a watchful eye, (if you are to go out, then yes seperate them cause your not there to see if anything happes) and if they come face to face...just sit and watch. There may be a few hisses and growls and maybe a few swipes but this is likely to happen in the first week or so. Unless it gets hugely physical or voilent, then you would have to step in otherwise leave them completely to it, so they can in their own terms suss each other out. At night...yes seperate them and only leave them together at night when the hissing/growling/swiping comes to a stop. All been well they should work each other out, but there are times when some cats simply cannot get on with eachother and never will..but the more common outcome is that they will tolerate eachother, like each other or adore each other.

The first few days will look as if the cats wont get on..but it's very common for some action to happen cause to eachother that cats is suddenly appearing in their face. Basically allow all cats to wonder the house freely but stay alert and watch to see when they come face to face or nose to nose then come night time seperate them until you firmly believe that no fighting will occur. At mealtimes feed all cats in the same room if you can, but place the bowls some what appart from each other so they can eat happily without having to see if there is a cat literally on top of them.

This is how i've done it and it's always worked, i strongly believe that cats will themselves sort it out and the outcome its more likely to be good then bad, but sometimes things just cant end happily and cats will never tolerate other cats, but i've never seen this in my household. And i don't think keeping a cat confide to one room for a few fays will up the chances of cats getting on. I strongly suggest you allow the cats to roam freely in the house and if they come nose to nost, just be on alert. After the first week...things will start to look up, maybe even sooner...who knows. If there are no hard on fights in the first day or so..then it's unlikely to go downhill, instead it will get better. Also as one stated above....check the history of this older cat and check if she can live with other cats first.