Is there a specific health benefit from sleeping in the afternoon. this is for my 3 year old sister.

Yes.: Younger children need more sleep to support healthy growth and development. They are growing at an incredibly fast rate (compared to older children). That's why babies sleep so much and why toddlers should get daytime naps.

Naps R Good!: Our brains do a lot of work during sleep and in young children an afternoon nap also helps promote brain development. Napping is pretty healthy for all ages. Some foreign countries encourage their employees to powernap after lunch. Shown to improve job performance and reduce stress. Just don't nap too long or it may disturb your nighttime sleep pattern.