Ottilia was the name we came up with initially, but I started having doubts as the 'ia' ending rhymes with DD1's name and both names have the same number of syllables, too. DH doesn't think it's a problem at all.
However, our (foreign) surname ends in a 'y', so matched with Ottilie, when you say the full name it sounds a bit .... hmm, 'not quite right' I guess.
Ottilia sounds made up - is it? But Ottilie is a name I recognise, IYSWIM. Having said that, I think two "ia" endings among siblings is fine, whereas something like Sally Barry (if that's what you mean re surnames) sounds less fine.
I prefer Ottilia, but I think it's better for her name to rhyme with her surname than her sisters name. Sometimes rhyming siblings work, but the only name I can think of is Lydia, and I don't think Lydia and Ottilia would be good at all.