You started your question with “under the legal limit, I might add”. The problem is that the legal limit is not quantified in terms of glasses of wine or pints of beer.
To drive legally you have to be able to pass an “Intoximeter” test at the police station which measures the alcohol in your breath (there are occasions when a blood or urine sample is used instead, but the principle is the same). The limit is 35 micrograms of alcohol in 100 millilitres of breath. That measurement does not equate to pints or glasses for the simple reason that there are too many variables which may cause one person to be under the limit whilst another, who has consumed exactly the same amount, can be over. Even assuming the amount of liquid consumed is identical, the strength of the drink, the amount of food eaten and when, the person’s metabolic rate, and even the weather can all play a part in how much alcohol enters the blood and how quickly it is processed and removed.
In short, if one of your in-laws is breathalysed at the roadside and fails and subsequently fails the evidential test back at the Nick, saying they “only had one glass of wine” will present no defence whatsoever. The price of failure is, of course, a mandatory disqualification for a minimum of 12 months over which the Magistrates have no discretion. As grown-ups the choice is theirs