Just because she is 2.5 doesn't mean she wont have troubles giving birth. Your sis might want to make sure she can afford any emergency vet bills that might occur (last time I spoke to a vet about a c-section, it was £250), and then there is the extra cost of food and litter while the kittens are growing up, plus worming, fleaing and vaccs for all the kittens. My first mum cat came to me when the kittens were 5 weeks old, they were getting through just over a 30l bag of litter a week, plus 6 boxes of pouches and biscuits a week. IF she can't afford this, then she does need to think of all the options, however hard, as it isn't fair to bring kittens in if she can't probide for them. Also, unless she knows the father, there is also the risk of sickly kittens if the dad was carrying anything like FIV or FeLV.