If you want to take over foodstuffs, the best thing to do is to look at the AQIS website to see if it's likely to be allowed. I did this the first time I went back to the UK to see what I could bring back and was pleasantly surprised.
I routinely bring back Bisto powder, Boursin (have it on the flight), smoked mackerel in a sealed packet, G&B drinking chocolate powder, vast quantities of milk & almond G&B chocolate (except last year because it disappeared), I've even brought back a kilo of dried cherries before now. I ALWAYS declare food and drugs (medications, nothing illegal!) - sometimes they are happy with what I tell them, other times they want to see but the only thing I've ever "lost" was an apple that I told them I suspected was in a bag but I couldn't find it - they x-rayed, found it, confiscated it for destruction - fair enough.
Rule of thumb - if it's uncooked/untreated animal or vegetable* product, don't bring it.
If they like Pimms then it might be nice to bring some of that over - it's viciously expensive here in comparison with the UK! You have the usual 2.25L allowance for alcohol to come into Australia.
Things you really should avoid are things made from any animal product such as bone, feather, shell; and any plant product such as wood, seeds, or flowers. Furniture (which you're obviously not going to bring) is ok because the wood is usually treated so won't be bringing any disease into the country; but untreated wood wouldn't get in.
Also, if you've never been before, make sure your shoes are all clean of mud - they get the arse over mud too!
Things I buy in the UK that aren't foods - books, DVDs, underwear - all much cheaper in the UK and better quality in terms of underwear. Also deodorant because I use a dry stick deodorant, and there isn't a decent one for women here.
Perhaps you'd do better to ask your brother and his wife what they'd like?
*using the wider definition of vegetable to mean all plants.