Sibling Suckling??

Hi, I have a 2 yr old male cat (Beary) and abt 3 weeks ago adopted a pair of 14 wks old female siblings.

I had to quarantine them for 2 weeks as they had the flu caught from the cat shelter. Naturally, my Beary hissed and smacked the door the first few days to show his alpha status.

On the 3rd week, they were are well and let to socialise with Beary. Surprisingly, the youngest of the siblings - Rani was very aggressive with Beary and would hiss and growl whenever he initiated play or just walking past her. The elder sibling - Ramya was quite neutral to the whole thing.
It can be that its just Rani's personality to be defensive/aggressive as she too treated Ramya with major hostility during a play which lasted for the whole night.

Ok, now another 'surprising discovery'. Ramya and Rani have been having diarrhea for the past 2 days due to the change in diet (Natural Balance) and so the litter (flushable clumping clay) is a total mess. So we had to put them in the carrier while we change the litter and wash the toilet.
While I was taking a breather, I heard a strange sound from the carrier... looked in and Ramya was suckling Rani!!

Is this normal?? I thought this was for very young kittens and between adult female cats.

Anyone have any idea??

Where is she suckling? If it is a nipple, it seems harmless. Sometimes tiny kittens get in the habit of suckling genitals, and that is very harmful for both.

You may want to stick with regular clay litter until the diarrhea is over-it can make a nasty mess, can't it!?!