Minecraft accounts linked
I don't think the accounts are liked, but instead I thing that your problem is that when someone joins a realm with active pack they will have to download said packs. if this is the case it means your son had to download the packs (for free, don't worry) when he joined the realm. In which case your daughters packs have just been victim to a glitch. I am just speculating so don't believe what i have said to be fact.