I think your BIL was a bit over dramatic TBH. Its not like you were 3 hours late for lunch. Its only half an hour. What if you were stuck in traffic or something and were half an hour late?
I try very hard not to wake bubs...I will leave it to the very last second as in the car is running, everything/one is in the car and then I wake bubs and go.
With three old siblings she has had to be woken lots of time and I hate it as she is a crap sleeper so if she is having a good sleep I really want her to sleep.
So...to answer your question...do I wake bubs? Usually I do as I personally hate being late. If its not really important, I will let her sleep. DD is not one of those babies that sleeps in the car or the pusher or wherever so it can be really difficult to get her to sleep if we are out.
And to answer another pp....yeah sure its easy to say 'just don't plan anything during a sleep time' but if they sleep at lunch time and more often then not people plan things at lunch then what are you supposed to do?
For me its more important to have a happy baby as it equals happy mummy. If I am planning a lunch and expect people to be there at 12 for example, all that person would need to do is say...'look cant make it at XX time but I will be there half an hour later...then I would simply say - will either save a plate or work around it. No biggie...I just appreciate being told IYKWIM.