anotherkate,
Thank you for your suggestions.
[name]Simone[/name] - You’re right, it does repeat the S initial, which I prefer to avoid. As for the name itself, it’s one I don’t care for. Maybe it’s the “moan” sound; it just sounds so solemn and melancholy to me.
I do like [name]Phoebe[/name], but feel I can’t use it due to my daughter’s initials (first name, last name) being PP (pee pee).
Spelling it phonetically (Feebee) just seems to ruin its charm.
[name]Celeste[/name] and [name]Charlotte[/name] - I love both of these, even though they do repeat the C initial (which one of my sons already has), but that’s not necessarily a deal-breaker.
I also love [name]Colette[/name]. I’m also partial to [name]Vivienne[/name], and even [name]Viviette[/name]. Not sure if hubby would consider these last three. (I won’t be able to actually discuss names with him until the baby is about to be born, or is already born. [name]Just[/name] the way he is.) But I want to have MY favorites picked out ahead of time. In the past, I’ve been lucky enough to have him agree with my (pre-chosen) first- and middle-name set for one of our children, and for the others, he has chosen several names off my list, and we have come to agree upon the final decisions together.
But anyway, 
I like the sound of Ya-neen-a and Yahn-a. But I would have to spell them with a Y in order to get people here to say it right.
Then, if I were to do that, would they look like made-up names?
[name]Nina[/name] - I have always liked this name. But I have a daughter named [name]Serena[/name], so wouldn’t the two names be too rhyming?
NEEN-uh, Seh-REEN-uh - I just think they’re too close in sound.
[name]Eliana[/name] - I really like this. What would you say is the ‘correct’ pronunciation?
As for [name]Katherine[/name] itself, I am not against using it (in the middle), but I think what I’d prefer to do instead of that is to use a more unusual variant of it, and therefore use it in the first name place.
What do you think of [name]Katerina[/name]/[name]Katarina[/name]? (Are these also too much of a rhyme with [name]Serena[/name]?)
As for [name]Angelica[/name] as a first name, my only reservation about it is that it does repeat the A initial (one of my four sons has an A-beginning name), BUT it is a form of my own name, so I think it would be acceptable. (Not that we have set rules about this.) 
I agree with your opinion that, since I do have so many children, most people (even many of my own family members, I think) will probably never realize that the new girl’s name is similar to her older sister’s middle name. And I think that’s the key word, too, similar (not identical). For those who [name]DO[/name] realize the similarity, I don’t think they would be bothered by it or think it was weird, because I think they would recognize why I chose to do that (naming both girls after myself, in slightly different ways). Besides, the older one with the [name]Angela[/name] middle-name is going to be about 9 years older than the baby, so again, I think you’re right.
So the set of sisters could be: [name]Serena[/name], [name]Marissa[/name], [name]Angelica[/name] (and who cares what their middle names are, for the most part?)
But you do raise a good point, that I will have to think about, and that is whether the third girl who isn’t actually named after me will feel left out?
Thanks again for all your comments and insights.
Also, anyone else reading this, please feel free to advise me with your opinions as well! 