Toddler Cousins

My niece is extremely shy and it takes her about an hour to cool off, she then turns into a completely different child, loud and eccentric. My LO is rather a neutral and outgoing one.

Now my husband has commented he'd rather not have them so much together because our niece's from one extreme to the other personality can make our toddler copy the behavior.

I'm not familiar with toddlers and their behaviors, we are actually first time parents. I want to know if what my husband says could be true and if we are just being in the way of their friendship?