Has it been confirmed if the two children slept in the same room?
If they did imo it makes the broken arm relevant in another way: it seems to make it less likely that it was a random stranger looking for a random child to abduct.
If there were two children to choose from, it would be easier for a random stranger to take the one with no cast on her arm because that child probably wouldn't need medical attention in near future and there is less danger that the uninjured child wakes up from pain and brings the whole house up screaming if you pick her up the wrong way.
Speaking as a parent whos 4 year old broke their arm over Thanksgiving - I whole heartedly agree. My daughter would scream out in the middle of the night in pain from her own movement. I cant imagine what she would have done if you tried to pick her up without her knowledge, especially when she had the soft cast on.