That is rather callous to rehome your older cats because you're fed up of them. But we don't know if that was the actual REAL reason for the sister doing it.
She may be in a seriously bad financial position, and cannot afford the vet fees of older cats. Our two are 17 and 15, and they have (between them) cost us nearly £1500 in various vet fees this past 6 months.
However, we stopped the pet insurance 10 years ago, when it went went up two and half times the amount, to nearly £60 a MONTH for the two. Se we have saved £6,000 in pet insurance in the last ten years. And that's not even taking inflation into account. Could be £7,000 or even £8,000 saved. So although it came at a bit of a bad time (with us having 20% less pay for 8 of the last 12 months with us both being furloughed,) we were OK with it.
But as I said, we don't know the exact circumstances of the sister taking the cats to a cats home, so we shouldn't judge really. She may have said she is fed up of them, but in reality, she may be struggling financially. She's hardly going to admit it if she is. Who wants to admit they're having financial problems?!
As someone said earlier, I am sure they will find a lovely new home with someone a bit older, who wants an older cat, that won't outlive them. And at least the sister didn't just strap a slab to their back(s) and chuck them in the canal!