Love Helena. DS was to be Helena. His now v elderly sister (in MN terms) is Grace, which of course is now way too mainstream.
Love Meredith - nn Merry - 'Merry and Nell'
Love Frances - although 'Fanny and Nell' doesn't have quite the ring for these modern times!
Helena and Maud - bear with me, 'Nell and Maudie', just lovely.
Helena and Miriam -Nell and Mim.
or there's the Shakespearean line to follow, as Helena is a heroine of his. I'll miss out Hermia, as she and Helena are sidekicks. You've got Rosalind already - lovely name, but it will be nicknamed to Ros. And you've used Beatrice. Some are perhaps too mainstream- Olivia, Imogen, Isabella...
But there's Viola, Portia, Hermione, Juliet, Cordelia, Cressida (reservations, just a horrid play!), Miranda (too 'flowery' perhaps, looking at your other names?) and what about Celia? Very much in the Helena area,namewise.