In the core family I didn't mind my children co sleeping occasionally, I thought it was nice and cozy, especially for the last baby.
When my exH and I broke up, the children would still come from time to time to sleep/camp in my room, especially the little fellow who was around 6 that time.
Things completely changed when I seriously dated my SO, who also allowed his daughter (6 at the time)who he has 50% of the time to co sleep with him!!She also was allowed to wake him up anytime during the night and made him to get ot off bed to do craft and games in the dark if she felt like it.You may get the picture, she was ruling his world, acting like a very entitled mini-wife.
When we started having regular sleepovers with our families, problems started to occur.Whereas my little fellow was having no problems sleeping in his room and bed, is daughter was far more resistant and made life absolute hell for month.
Being overcompensated and babied all her life she was not prepared to give up her status that easily.
We had attempts of all kinds, letting her sleep with us half of the night at the beginning which made me feel completely uncomfortable, him following her to co sleep with her in her bed, he sitting and coddling her half of the night .
It was completely insane to a point where I was telling him that it has to stop entirely and he needed to put his foot in the door or there is no adult rs possible.So he started working on it and insisting that she sleeps in her room and in her bed.She tried making up deseases and to manipulating him to feel guilty but he finally with a lot of talking and praise she stays now in her room.
See , that 6 year olds fears can be serious and the adjustments are hard, but co sleeping is difficult in a step family and not necessary helpful.
It would be better for everybody involved to create a reliable safe pattern at night, install a night light and maybe a invest into a special "big boy" toy as a reward for staying in his room.When he wakes up, mum should go and comfort him quickly but not getting dragged into spending the night with him.The little boy needs to learn that he is very safe and loved but also has to accept that his mum needs to sleep with her husband at night and not with him.The earlier he understands, the better for every member in the family.If he understand that his "fear"is recognised but not rewarded with extra attention during the night, he may manage to let go of it soon.