The new little brother and the new bed came at the same time and maybe there's some insecurity with all that change, though the bed has been there a while. Did she possibly fall out of it once and that's scaring her?
For young kids, things that are familiar in the day can take on scary properties in the dark, even with --sometimes especially with -- a nightlight. That blanket folded over the back of the chair can become a frightening blob once the nightlight's on it. Stuffed toys that are fine by daylight can cast weird shadows. Even my 10-year-old occasionally says that dark corners in her room are worrying her and she needs more light in there at bedtime.
See if you can talk with your granddaugher in a way that gets information from her but doesn't plant scary ideas in her head. (Not always easy!) "What is it about your room that's scary? Can you tell me?" She may just say "I don't know" or she may be more specific. Does she have a nightlight? Have you tried different or brighter nightlights, or putting them in different places?
Does she fear that when she's in her room alone, her parents disappear (common belief among very young kids-- she's old for it but there could be traces of that going on), or they might leave the house without her?
Is it the bed itself, mostly, since she was interested in the toddler bed? She could feel she's adrift and/or too high up in the double bed and a smaller bed could help. You could just put a mattress on the floor, which could make her feel more secure. (I'd move out the double bed for now, if you do that -- its underside will create another scary shadowy place if she's sleeping on the floor and the bed is still in the room.)
Also, be sure she spends happy time in her room during the day and that it is not just a place she goes into at night. Some kids really don't play much in their own rooms if they have the rest of the house for play. Play with her in her room in daytime and early evening before bed and show her how nothing's in there at night that wasn't in there during the day when she was having fun there.
This may be a phase where honestly someone just has to sleep with her sometimes. That's not wrong and doesn't mean her mom or dad will still have to sleep with her for years to come.
Let us know what happens. I hope it works out.