I love the name
Diana - it's one of my favourite names, though I would spell it '
Dianna'. I tend to imagine someone very book-smart, but also confident and friendly, with fair hair and green eyes.
David is also quite a good name. It's the name of my cousin, so I don't really tend to think of a particular sort of person other than him when I hear that name.
Agnes I'm not so sure about. It's not a terrible name, but I think it does have a tendency to sound a bit 'little old lady-ish'. I certainly have trouble imagining a baby
Agnes (though I suppose as a baby she could be called
Aggie, which fits a little better).
The MNs are both really good by the way, especially
Beatrice (though I personally lean slightly more towards
Beatrix).
As for sibsets with names starting with the same letter, I think whether or not they are too cutesy depends largely on what names you choose and what 'vibe' they give out (if you know what I mean). So while
Harriet and
Henry is a little bit twee (both old-style, traditional, upper-class names),
Hannah and
Harry would be fine (one's Hebrew and one's English). I can't really explain what I mean any better than that.
I hope my comments were of some assistance to you.