The topic is interesting and it is actually not as hard as you thought because it's about YOUR family, a very familiar thing with you.
As the topic requires only 1 difference, so it's a bit hard. I have some sugguestions for you.
First: Dad and Mum have difference in:
+ Gender: This effect strongly to how ur dad value family and his job, same to ur mum.
+ The way they grow the children: Mum is gentle, dad is strict, for example.
Second: Bro and sis or between the kids
+ The dependence in parents: the bigger one is more independent, therefore, he/she is more mature, blah, blah
+ You even can talk about how the housework is shared (but this one is not really good)
Third: Parents and children:
+ A very huge difference due to ages. I don't have much idea about this!! Sorry.
Hope what I say is understandable. Enjoy and Good luck.