vavanopi8355-san,
kon'nichiwa!
It's quite good, actually!
I think it'd be the best if you say,
あなた達は仲良しだね。いいね。
If you say this to your close friend, you can say that in very casual way. So, instead of ~だと思う, you can
use just ~だね
And, instead of よかったね, いいね sounds better. よかったね is rather your opinion on past fact (something happened),
but friendship continues, and people usually say "good for you" about friendships with a certain "envy" or "admiring"
feelings, right? いいね expresses all those nice feelings (even if it contains enviness, it's a positive feeling, not negative).
Hope this helps!
Natsuko(奈津子),
Team JapanesePod101.com