R.,
Red flags pop up with your word, "suddenly". Yes, it's normal for 5 year olds to go through stages of feeling insecure without mom and dad around, especially without the presense of other kids for back up. Age 4 is known to be a year of intense associacation with peers, so much so that if there aren't other kids present, she could be feeling 'lost'. You didn't mention if your sister has kids your daughter is comfortable with. I know you just need to get out from time to time, but as a mom, I would be sensitive to your daughter's feelings at this time. If it's just a phase, she may calm down as quickly as you perceive she came into this. If something bad has happened with an adult in her life (and I pray that isn't the case!), she also may be manifesting that into all the adults in her life she has trusted in the past. Stay open to dialog with her, so that she can tell you what she's afraid of eventually (hopefully). In the mean time, exercising understanding and protection in response to her fear seems the safest thing. Maybe for now, you can look for trustworthy friends with kids her age for babysitting.