I almost feel like I painted myself into a corner naming my first two [name_f]Monet[/name_f] and [name_f]Mirabelle[/name_f], because now I have to find a name that is complementary, yet not too matchy, and I have to struggle with the issue of using an M name or not for the third. We want a name that is accessible and pronounceable in [name_u]America[/name_u]. Obviously I don’t mind unusual names, but I prefer them to be “familiar,” in that someone would at least have heard of it before, even if they don’t know anyone else with that name.
If you plan on having other children, I don’t think you have to choose an “M” name. If you don’t think you’ll have anymore children, then it would be ok to have the same initial as long as the names are different enough. However, if you really can’t find an “M” name you [name_f]LOVE[/name_f], then break the pattern! No one is going to arrest you!
I agree with everything @mischa wrote about using M name. From your list I like [name_f]Aurora[/name_f], [name_f]Georgina[/name_f], [name_f]Margaux[/name_f] and [name_f]Sabine[/name_f] with [name_f]Mirabelle[/name_f] and [name_f]Monet[/name_f].
I think you should stick with a M name - personally it bugs me when themes or letters aren’t applied to all the names in a sibset. All the M names on your list are great, because they all have a ‘Mah’, ‘[name_f]May[/name_f]’ or ‘[name_f]Mag[/name_f]’ sound which means that all your children will have names that sound quite different. I like [name_f]Margaux[/name_f] or [name_u]Marlowe[/name_u] the best, but I do like all the M names on your list!
To be honest, the first name that popped into my mind when I saw the name if the post, was [name_f]Melisande[/name_f]. All [name_m]French[/name_m] names starting with M, all unusual, but familiar and easy to pronounce, and all uniquely beautiful. I would stick to the [name_m]French[/name_m]-European theme at least (I dislike breaking up themes).