Do puppies out of the same litter grow at different stages because the sister of my boy is so much bigger! they are 8 months ????

My boys litter brother weighed 14Kg more than he did at one point.

My boy is from a litter of 8(4 of each) and so far I've seen all but one( a sister). Idõ is bigger than one of his brothers, pretty much the same as one(the other I haven't seen since they were 5 months and they were the same then) and bigger than all but one of his sisters.
Theorectically they shouldn't vary a huge amount once they are 'done' but it does happen, our girls are aunt and neice(her mother being the others sister)and they are very different hence why our girls look nothing like each other!!

hi - my labradors last litter is now 14 months old- we kept the smallest dog out of them all- and now he is the biggest- 35 kg!!! his siblings are very varied-
but she was put to 2 stud dogs so could have had pups from 2 different sires if thats possible?
i think its quite nice to get a varied bunch of pups!

Yes s mixed litter from more than one sire is quite possible, but why woudl yu do it.  You woudl need to DNA test all the pups in order to be able to register them.  The aim of most breeders is to produce uniform pups.