As everybody has said, there is only 13 years between them which really is a very short period of time so it isn't surprising that they look more like sisters than mother and daughter. Plus of course, the fact that most 16 year olds look older than they are with make up.
Everybody brings up Lucy Fallon being older than Bethany but remember Tina O'Brien is also older than Sarah Lou so the age gap between the actresses works out much the same.
If I didn'( know the history, and was seeing them for the first time, I probably would double take at Sarah being Bethany's Mum, or wonder why they'd cast such a young looking actress as the Mum. But having seen it from the start and knowing that Sarah was 13 when she was born (and the actress actually 16) I don't have a problem with it. Sarah just still looks young for her age, her daughter looks like a normal 16 year old with make up to me.
As this subject has come up again, did anybody else notice that the actress originally cast as Bethany and fired when they discovered she was mid twenties rather than 19, Katie Redford, was Rose in the 'Keeping Up Apperances' prequel on Friday. Very good she was as well.