Help - cat behaviour question

[Attenborough]

We foster cats for the local shelter and have picked up a new pair today - tabby girl and tabby and white boy. There's some uncertainty as to whether they're littermates or mother and son - he is a little bigger than her. Both are 2/3 years old.

We've done the usual settling-in routine but as they're pottering around exploring, she occasionally growls at him, and we've had one hiss. This happens roughly every 40 mins.

Though we've had dozens and dozens of cats in all sorts of combinations, we've never dealt with this particular behaviour before. Should we just ignore it and presume they'll work it out? They seemed perfectly ok to be in a carrying box together earlier today.

DH and I adopted two cats from a shelter last year, approx 2 years old, sharing a cage in the shelter (oh good we thought, they get on well then!) and we were told they are littermates.

We got them home, let them loose and they scooted off in opposite directions to explore the house. Then after that they were quite inclined to hiss and growl at each other quite frequently. The smaller of our two cats generally seemed to be the main agressor too.

I was getting worried that we might have to send one back to the shelter if they couldn't work out how to live with each other in relative harmony, but didn't really know how intervene to make it better between them because this was behavior I hadn't come across before either.

After being really hissy and narky and generally unable to be in the same room with each other for about 3 days the agression levels just tapered off, and within a week they were absolutely fine with each other.

They do occasionally still get grumpy with each other, but it is always with a reason now rather than just random horrible growling!

They are currently sharing a sofa and sleeping peacefully :)