Mysterious Floating Green Triangle

This isn't my story, it's my sister's. This happened when she was about 11 years old, (she's 18 now).

She was over at her friend's house, let's call her "Cindy", and they were lying on the trampoline, it was a sleepover, so it was dark outside. Cindy had some neighbors that were her age, let's call them "Steve" and "John", and they were also outside that night.

My sister and her friend Cindy were discussing UFO'S and were laughing at the thought of actually seeing one, they didn't know that that very thing was about to happen.

My sister looked up and went pale, she managed to nudge Cindy and point in the direction she was looking, and when she looked both of the girls stared in amazement, there, about 3 feet above the ground was a floating pyramid, it was a metallic green color and it almost seemed to glow.

It started to move into the front yard (they had been in the back) they looked up to see that the two boys had seen it too and had run into the house. Cindy managed to get the courage to follow this triangle so my sister followed, as to not look like a coward, and this thing went across the front yard and out onto the street. It moved down the street a couple of blocks or so, turned down another street, and it was gone.

My sister and her friend never spoke of the incident again until about a year ago, they were over at Cindy's house again and the two neighbor boys were over there and the four of them got to talking about UFO's again. Steve suddenly said, "I saw one once, it was about 6 years ago, I was in my backyard with John and we saw a floating, glowing, green triangle figure moving around between our houses, it seemed to be only 3 feet or so off the ground. Do you remember that, John?" John suddenly seemed to look very scared "Yes, I remember, we ran into the house and found my mom balled up in the center of the living room crying and moaning as if something utterly horrible had happened, and we couldn't get her to tell us what was wrong, she just kept mumbling until we went to bed, and the next day she acted as if nothing had happened...."

Cindy and my sister looked horrified, they remembered that they had followed the thing and seeing it, it was exactly as the guys had described, and as they kept discussing it, it turns out that this had happened the very same night my sister and Cindy had seen it.

I don't know what this was, and neither does my sister, Cindy, Steve, or John. If any one has any idea whatsoever or has had a similar experience to this, please e-mail me.