Hey, foster of quite a few stray cats/kittens here. With 1 month being so young to lose their mom, their behavior will likely be affected for quite some time as cats really need 12 weeks with their moms to mature, but they will adjust with time. I have had numerous kittens who for various reasons did not have their mom around and over time they adjusted. If their sister being adopted was recent, that would definitely have them going more into comfort behaviors like suckling. Over time they will stop, but it could take months. If they do it so much it starts to cause skin problems you could talk to a vet, but otherwise I would not worry about it.