Babies holding babies

As the days of pregnancy close and motherhood right around the corner, I've noticed I worry about a lot. But this is particular. I have a 2 year old niece and a 6 month old nephew. They are great and everything but the 2 year old always wants to hold her brother and always has been like that. Granted her mother lets her, but it really worries me. Allie( the 2year old) is super rough and I'm worried about her holding my baby when she's born. And I know her mom will let her. I would rather her just look than maul. Advice? Should I just let it go and be nervous or not let her hold the baby and potentially cause hard feeling? I'm a ftm too