Yes, if you find Hannah (and her son, of course) in censuses earlier and later, that should sort it out.
If in 1851 and 1841 Hannah is Hollis, then that's her maiden name.
Finding both Hannah and her son in later censuses can help narrow down her birth year.
If you find Hannah with her father as George Hollis, it doesn't imply incest, but that William used his name later as father, either because he was brought up as Hannah's sibling, because he thought of George as his father rather than Grandfather, or simply because he didn't wish on marriage to draw attention to his lack of father.
Don't fret, we all have problems like that encountered at an early stage.