We fostered a litter of kittens and the mum who were stray. We kept 2 of the kittens and rehomed the others. They are lovely and part of the family, 2 weeks ago one of them disappeared, totally out of character, we’ve looked high and low she is micro chipped but no sign. We are all very sad. They are 20 months old. The one left behind is really sad, she spends the night calling her litter mate and sits outside all day refusing to come in. Is there anything we can do to help? Does she need another cat or will they not get on?