i think they are really clingy to him because he has been overseas, and they dont see him much. Maybe they're scared that something can happen and they won't see him again.
I dunno being the youngest daughter, i have an older brother, and my mom is always there for my brother. so I can if he left or something overseas she would be freaking out.
I think when your boyfriend comes back for good, it will be wild at first, but then everything will settle down and the clingyness will go away.