FWIW, my DCs and step DCs attend clubs like this together. It's meant that they go off camping at weekends together etc. It has brought them much closer together, and they usually CHOOSE to sleep in the same tent as eachother.
If they were full siblings, you wouldn't even be thinking about this I don't think. I think (in the nicest possible way) you may be projecting your own worries onto DD - worries that she just doesn't have).
Dd sounds like a nice generous child so ask her and go with what she says.
At rainbows they don't have sixes or patrols like brownies and guides, but you could ask the leader to put them in different groups if they do group activities. Stress that this is to give them space like you might for any siblings very close in age, not because they don't get on.
Dd 5 goes to rainbows, she really enjoys it. Before summer she was taken there by after school club and we picked her up on the way home from work.
Dsd also 5 really wants to go to rainbows too, she couldn't go with Dd as they go to different schools and dsd doesn't use after school care as her mum is not working at the moment.
In September I am changing my hours which means I will be taking Dd to rainbows and picking her up myself. If dsd's mum dropped her off with me on the way home from school I could take her too, give them tea afterwards and then drop dsd off before bed.
Dsd's mum won't organise for her to go to rainbows near her because it requires too much effort on get part but I think she would jump at the chance of dropping her off with us for an extra evening.
However Dd already has to share her room and toys and friends with dsd and dsd joined her swimming class because her mum wasn't taking her to her lessons. Dd has never complained and refers to the room as their room which it now is and they get on well. It just seems Dd never gets anything to herself anymore.