I have never seen this before between two cats. It is absolutely charming. It seems that the senior sister is comforting the junior sister. It’s quite amazing to look at it and perhaps the stress of being at the shelter in a cage resulted in this behaviour. We are told that they have been rehomed together which is wonderful news.
For me, it reinforces my belief (and what is regarded as general knowledge now) that cats do have quite advanced emotions and this should be recognised by everybody as it should enable people to respect the domestic cat more. Greater respect leads to better cat welfare.