Well, if
Helena is that meaningful to you, then I'd say it's not the end of the world if
Serena and
Helena look similar. After all, their names would be pronounced differently and would be rarely listed together. I don't really think
Serena and
Helena are that close based on the way you're planning to pronounce them. Then again, I have a dd named
AnNA and would likely name another dd
MariNA and that doesn't bother me one bit.
Personally, if
Helena had a lot of meaning to me, I'd let my dh have
Serena if it made him happy. Unfortunately, my dh doesn't care for
Corinna or
Serena. :-(