Yes, this is my experience as a mum: my dh began encouraging ds to try a sip of bitter at around that age. I don't consider it to be harmful. All it does is let the child confirm for themselves that it tastes really horrible and they don't want to drink it.
I was OK with this, as it's the way I was brought up. I was even given a very small glass of wine (think shot glass) with Sunday lunch from about that age. My parents saw it as teaching me to drink responsibility in a social context.
I still drank enough to be uncomfortably drunk to the point where the room spun and I vomited a couple of times as an older teenager but then we all have to find our limits and I never did it again.
I almost never drink now as an adult. I don't think it's compatible with parenting but my dh will drink an absolute maximum of 2 pints of beer one night/week when socialising at the pub.
I think that your own attitude to and history with alcohol matters a lot:
If you have family members or are yourself an alcoholic then maybe you fear what happened to Drew Barrimore. Maybe your child does have a pathological potential for alcoholism. In which case, I think you are perfectly justified in asking everyone to keep alcohol out of their reach. Ask them to think about it like a food allergy, except there's no epipen and it could wreck/shorten their life considerably. You are not being unreasonable and should talk to your child in an age appropriate way about why alcohol could be unusually dangerous for them.
If you are teetotal for religious reasons, again you are not being unreasonable in asking people not to give your child alcohol, maybe in the same way as you want adults to discourage your child from swearing or telling lies? I don't know. This isn't my background but I still think it's entirely reasonable and you should talk to your child about your beliefs too so that they understand why they are not offered alcohol.
In both cases, it is quite reasonable to ask uncle to stop allowing your ds sips of alcohol and not allow ds around them unsupervised if you don't trust them to respect your decision.
If it is just a vague feeling of unease with no ethical, religious or health concern then, gently, I think that you might be a little overanxious and you can relax on this one.
Maybe just stipulate no spirits as I think that could genuinely be a health issue.
My elder db was given a dummy dipped in whisky if he couldn't sleep (different times!) and unsurprisingly has a glass of whisky every evening before bed!
As an aside, two generations back an evening glass of whisky was legitimately prescribed to my great uncle by his Harley Street consultant as a cure for stress!
Sorry for the two slightly off topic family anecdotes.
I think you just need to think through your reaction and your reasons for it and act accordingly. If you feel that you do need to forbid the occasional few sips of beer for health or religious reasons, then get your husband onside and make it clear to your relatives. Otherwise, relax, it's really no biggie.