How important is active cat playmate for a 1 yr old cat?

My situation is pretty specific so let me include all the details.   I know this isn't the place for advice on the medical parts of this (unless someone does have that expertise then terrific.)     But my primary question is about how important is is for my 1 year old cat's mental and social development to have a playmate his own age.

For years I had 2 cats the same age.    They are now about 8 years old (best guess for rescues).       We had debated getting another cat for some time so that there would be a younger cat to be around for years before we had to look at losing one of our original 2 do to aging.

Then last summer we ran across 2 full sib rescue kittens that stole our hearts.    We adopted them and with careful work, integrated them with the other cats.    The adults were good with the kittens (grooming and such) but didn't play much with them.   But that was ok as the kittens played non-stop with each other. 

Then a month ago, one of the two kittens passed away from FIP.    Now we've done lots of research and work with our vet and are well armed with info about the situation health wise.   And we will wait the appropriate amount of time regardless for both grieving and health.

The question now is that we have a super active 1 year old (Vex) and 2 only moderately active 8 yr olds.    The 8 year olds will chase or wrestle each other several times a week, but will only play swat with the 1 year old about 1-2 times /week for fairly short duration.     One adult does groom him a few times a week as well.

I play with Vex in the morning for 10 min of very active play, and he gets lots of other attention as I get ready for work (petting and talking and tossing the occasional toy).    And he gets another 20 min play session each evening (high to moderate activity), and lots of various forms of attention all the time from me.   The adults occasionally join this session, but only for very short times) The above listed attention is what he gets from the other cats.

But he used to play non-stop with his brother.   And when they weren't playing they often slept together.    So it becomes a matter of how important is it for his social and mental development for him to have a playmate his own age/energy level. 

Since he was a full sib, and his brother died of FIP, that puts him in a higher risk bracket every time he is exposed to another strain of the Corona virus.    And bringing in a new cat from a shelter or cattery (I live in Alberta, Canada) they will almost certainly have the virus.   There is almost no way to avoid getting a cat with a virus without finding a rehome from a single cat household (which is hard to find).   Especially when we would look for a cat that loves to play with other cats.

Every day we mull the debate of how important it is to have an active playmate for Vex, vs the risk of bringing another cat into our home.    And I know only we can evaluate that risk.   But we just don't know about how adequately his needs are being met by his existing situation.

So we would love some thoughts on how much he does or does not need a playmate his own age.

Thanks so much!

First, let me say how sorry I am that you lost one of your boys to FIP.  That means he was only about 11 months old and almost into the "safety" zone too (one year).  His brother is probably not going to develop it, but I do know that awful feeling while you wait and see, after losing one.  Our group lost two, four months apart -- one at age 4 months, shortly after being spayed -- and her three littermates never got it even though they were with her the whole time.  The next one was at age 8 months, in early January, not long after being in Petco for a couple of weeks with her whole family too (2 sisters and mother).  The mother and two sisters have not come down with it, nor has any other cat in the group they are with.  In both cases, it was one out of four.  In both cases, they were under a year old.  In both cases, they had a previous stressful event that precipitated it (spay for one, being on display in Petco for the other).  In both cases, the other littermates had the same stressful event at the same time, plus they spent the whole time of illness still with the littermate, and did not come down with it.  Which is to say, I do NOT think he is more likely to come down with it just because he had a brother who did.  I know of at least one other case similar to this where a brother and sister were neutered/spayed at the same vet at the same time, and a few weeks later the male developed FIP and the female never did.  

I think he would like to have a playmate, and the thing about FIP is that it is so weird you can never tell who will get it and who will not get it, but usually it's the cats under one year of age, or immune compromised cats.  In your situation, if one littermate died of it, the chances are good that both kittens carried corona virus (most shelter cats do, but it doesn't turn deadly in most of them) and so the one you have there is probably not going to develop it if he hasn't done so already. Getting another cat a year or older from the same rescue might be a good idea, better than getting a cat raised privately who was never exposed to corona virus.  There are never any guarantees, of course, but the odds are better if the cat is over a year old, and he's probably safer if he DOES come from a multicat rescue situation, especially if he has been there awhile and has not developed FIP.

This is just my opinion, of course, based on experience and a lot of reading on FIP.