My sister has a wierd problem with her foretoe. while she is doing "nothing" her toe suddenly locks like a cramp and it doesn't gets normal quickly.

Cramps: Cramps most commonly occur when a muscle is used beyond its endurance capacity. In an attempt to prevent the muscle from being used any further, the muscle locks up to prevent ripping or tearing. You need to figure out what she is doing to overuse the foot muscle and avoid overuse. Additionally, an endurance building exercise should be started to prevent this in the future.