There are a few options:
1. Create an instance variable - AttachmentPoint=1. Use it to identify which image point to attach Sprite2 to, and increment it by +1
2. If attaching with hierarchy you can simply check the number of children on the parent sprite.
3. You can loop through all 4 image points and check if any of them is not overlapping Sprite2. Use "System Is overlapping point" condition.
Right, I should have said this before: I've just started dabbling in Construct, but I think Number 1 seems simple... I feel like I might be missing some context: how do you identify the points with an instance variable? Is that where the Event Sheet comes into play?