Help needed so bad *cries*

Myself and my partner live together and on days off we usually have his Mr2 and Miss6.

Miss6 is an angel and god couldnt have gifted this family with a more well behaved child. Mr2 is still being nursed by his mother and theres no structure at home, therefore when he comes to stay with us he is usually beside himself.
Lately he has been clinging to Miss6 ever so much, holding onto her and not letting go whilst crying and crying. We understand the situation he is in, and i sympathasise with him, but how do we stop him clinging onto his sister so much as it bothers her and stops her from enjoying herself.

He also cries really bad to the point he is beside himself come bed time. We just cant settle him.
We like to have alot of structure and healthy discipline in our house so its hard when its the opposite with their mother. Hes also moddycoddled at change over which makes that quite traumatic for him too.
Any advice id appreciate.
Thank you