My dd (22m) has banged her head quite badly about 3 times and I haven't (yet) had to take her to A&E. The main thing is get a cold compress straight on it and make sure the were no signs of passing out. Like your BIL says keep an eye on her. I'd keep checking in on her tonight whilst she's asleep. Also another thing someone told me is that it's good to get a lump as it means the swelling is coming out of the head away from the brain, but I don't know how true that is.
Hope everythings OK, I'm sure it will be. They are tougher than we think. I also think worrying is contagious, once one person starts it spreads round. Just keep checking her for your peace of mind.