I love
Margaret! My last name starts with Mar-, so I its not very high on my list to actually use but I just love it!
I love the combo
Margaret Anne as something simple and elegant. Other ideas:
Margaret LillianMargaret ClaireMargaret LucyMargaret AnnabelMargaret RoseMargaret EloiseMargaret JaneMargaret FrancesMargaret OpalMargaret Fern
For a sibling, I would do something classic like
Katherine,
Anne,
Jane,
Rosemary,
Clara,
Mary or Edmund/Edward,
Robert,
George,
Charles,
William etc.