There are quite a few organisations who will take on your pets after you "fall off the perch".
DON'T go to the RSPCA. They used to have a good program, which they decided was obviously too generous, so made it so expensive you have to be a millionaire to afford it now, and because they are charging a huge amount per year per pet, most people couldn't afford them, and estates would be held up for years and years if the dead (RICH) person left a kitten behind!! Just stupid.
After weeks and weeks of searching on the Internet, I found a no-kill shelter near where my sister lives in Qld (SCARS - Sunshine Coast Animal Rescue Society) who was willing to take on any of my cats which outlive me - I make a tax deductible donation to them of $25 per month at present, and have bequested them with a certain amount per cat after I die - they will be responsible for the cats for the rest of their lives, although I've given them my permission to let the cats go to carefully selected foster parents, so long as the shelter stays in overall control of their welfare.
Since I approached them and got this organised, they've established an official Legacy Cat program. This place is good for me as it is right near where I will be living at some point in the future. I have also bequested them the same amount that I am leaving them per (one) cat should I die without any cats left behind, just for being good guys!
The Animal Welfare League seems to have quite good programs - the last time I looked at their sites they didn't ask for a specific amount to be left to them, but obviously a generous bequest would benefit yours and other cats as well, and is the right thing to do.
If you decide on a place that you'd like your cats to go, know what you would require for them (i.e. no fostering/fostering OK, keeping all the cats together, whatever), correspond with them by email to find out what they require of you insofar as the appropriate wording in your Will, get your solicitor's opinion and then change your Will.
It's actually quite easy, once you find a place that you like.
I have a few nieces who love my cats, but - no offence to them at all - I don't believe they would look after them as well as a shelter that is being supported by me and who will receive a large amount of money to care for the pusses as per my requirements.