Just curious. My lab is almost 8 months. One of her siblings has started her first season. Do siblings usually start at a similar time? Just so I'm prepared as it's quite early?

Sometimes yes, sometimes no Often different lines have trends, some lines may generally be earlier starts to season and more frequent cycles, some are later starts and yearly cycles, but all dogs are individuals so there's really no way to say for sure.

We kept a couple of litter siblings almost 4 years ago, they both stated their seasons, 1 at 9 months and one week; the other at 10 months.
Their individual seasons vary between , 33 to 44 weeks and the other 39 to 40 weeks. They have just had their seasons this December 4 days apart.

With my last few bitches when I had more numbers the last few started their seasons later than their sisters by upto 3 nonths

I did wonder whether the adult girls presence suppressed it???

The only thing you can say for certain is that if one has started her first season, any of the siblings could start at any time! 8 months is about average for a labrador-sized breed so check her bedding every morning (maybe cover it with a white sheet to make it obvious ) for signs.

Thanks for the advice everyone. She has gone from sleeping really well at night to a couple of nights of barking (she is now in our room so fine again!) so I did wonder if it could be the change in hormones unsettling her.

I'd say yes, but of course, it's known that bitches can bring each other into season, siblings or not.   My bitches, except one, usually started by 8 months, and then cycled at that interval, usually.   And of course, having litters does alter cycles somewhat.  I had just one, who was a singleton puppy, who unexpectedly came in bang on 6 months and then started cycling at 5 monthly intervals.  I had her spayed - she wasn't really up to my standards for showing but she stayed with us her entire life, regardless.