It shouldnt by ill will. But accidents may happen, type it wants to play... Or if it still wants to nurse mom, and being much bigger he will of course maltreat the smaller kittens.
But it will not harm them on purpose.
(but mom will perhaps not be disturbed nor pested when she will be in peace together with her small kittens - so it is often best for all parts if mom and her nursing litter is for themselves)
you were right the kitten wont harm the newborn kits but I have to supervise when she's nursing because he tries to nurse also I have had to pull him off her a few time he tried to nurse but mostly he snuggles in behind her and sleeps