not really unreasonable, no - are they going to be buying you presents as well? Has your mum a general rule that anyone who comes to dinner has to bring presents for everyone else?
I'm not suggesting that you should only buy in the expectation of receiving - more that if they DID bring you a present, it might be a touch embarrassing not to have one to give to them in return.
Chocolates is a good idea - or biscuits.