By definition, 1 "drink" is defined as:
1 shot or drink of 40% (80 proof) liquor containing 1.5 oz. of that liquor
OR
1 glass of wine, 9 oz. in volume, at approximately 6 to 7% alcohol
OR
1 beer, 12 oz. in volume, at 5% alcohol
This definition allows for the easy calculation that each drink raises the person's blood-alcohol content (BAC) by 0.02, on average. Smaller people's BAC goes up faster than larger people's BAC, on average. Some people have a developed tolerance. Some people process alcohol better than other people for numerous reasons, only some of which are genetic. Men process alcohol faster than women, and therefore the BAC of men goes up slower than women, on average. Any one of these factors may trump another in terms of what most significantly affects a specific person's rate of raise in BAC & processing of that liquor. In addition, if the person has just eaten (within an hour to 2 hours) their BAC will not raise as quickly. However, all these things considered, each "drink" raises BAC by about 0.02.
A person's liver can process 0.02 of alcohol in about an hour, with individual variability determined by factors stated above. In addition:
Some mixed drinks (e.g. a rum & coke) are "tall" instead of "short," and likely mean that they contain 2 shots of liquor, or 2 drinks.
Some shots are 2 oz., and would then be equivalent to 1.33 drinks.
Some beers are 7.5% alcohol, and would then be equivalent to 1.5 drinks.
Some beers are 10% alcohol, and would then be equivalent to 2 drinks.
You get the idea.
So now, let's calculate your family member's BAC. Let's say they have 3 drinks across 1.5 hours. Let's also say that the first drink was not "chugged," and was completed 30 minutes into the bar-stay. So it's really more like 3 drinks in 1 hour. Therefore, that's a 0.06 BAC-worth of alcohol, and 0.02 of it would likely have been processed, leaving them with a BAC of 0.04.
0.04 is not enough to make them a trainwreck waiting to happen. However, I would also say that it is the maximum threshold before a person is too impaired to be a competent driver. For reference, in my state (New York) 0.08 is full-on "drunk" & gets you a DWI.
I highly recommend going to the bar with your family member and monitoring how they truly drink. Is it actually 3 drinks like they say, or is it more like 5? Are they small mixed drinks or talls? Look at the percentage of alcohol on the beers -- is it 5%? 6%? 8.5%? How long do they stay there? 1 hour? 2 hours? Take all these things into account and have a breathalyzer ready. If you see activity that points to your family member leaving to drive with more than a 0.04, tell them to blow into the breathalyzer.