Our DS is called Heath. DD is due shortly and the only name we've found that we both really like is Thea.
I think they're too similar, given that Thea is basically an anagram of Heath, minus one letter.
DH thinks that that, whilst the letters are obviously similar, they sound completely different, so it's fine.
Thoughts? Would you find a brother and sister with these names a bit odd, or are they different enough in sound (as most people hear names rather than see them written down) that you wouldn't notice?