Old School Anime

I've been searching for years now for a particular anime that I saw when I was a little kid and I need some help. The thing is I was like 8 or so when I used to catch episodes of the anime on TV(21 now) but was never able to watch the entire series and thus I don't remember the name of the anime but I do remember some distinct details.

1. The anime was a military/fantasy/mecha Anime
2. In the opening scene of the intro there was a cannon that fired a shell.
3. The protagonist of the anime has a crystal/diamond on his forehead that he uses to transform into a mecha style warrior
4. He also has a brother/antagonist with the same ability
5. The war was between the earth and space
6. The anime style kinda reminds me of the way Sailor Moon was drawn

If anyone has any leads, I'll seriously be eternally grateful

OP, its clearly Boku no Pico.

Good job, but help me please!
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