I love [name_u]Piper[/name_u] and [name_f]Rhea[/name_f] is sweet but I would go with [name_f]Sylvie[/name_f] (even though it isn’t my favorite) just because of the sibilings. [name_f]Tessa[/name_f], [name_u]Theo[/name_u], [name_u]Sloane[/name_u] and [name_f]Sylvie[/name_f] 
Everyone gets a buddy. (Unless [name_f]Tessa[/name_f] and [name_u]Theo[/name_u] are twins)
And as long as you stick in a strong middle name [name_f]Sylvie[/name_f] [name_u]Murphy[/name_u] is not too out there
I would be hesitant to add another S name, since you already have a [name_u]Sloane[/name_u], but that’s just my personal preference.
I think [name_f]Rhea[/name_f] [name_f]Katherine[/name_f] is beautiful and [name_f]Rhea[/name_f] complements your other kids’ names nicely.
Not a fan of [name_u]Piper[/name_u] at all.
Usually I don’t like first name and surname with same end sounds especially with same number of syllables but I actually think [name_f]Sylvie[/name_f] [name_u]Murphy[/name_u] kind of works. And I adore the name [name_f]Sylvie[/name_f] and think it is a million times better than [name_f]Rhea[/name_f] and [name_u]Piper[/name_u]. I also adore [name_f]Frances[/name_f]. [name_f]Francesca[/name_f], [name_f]Helena[/name_f] and [name_f]Camille[/name_f] are also good choices.
[name_f]Sylvie[/name_f] [name_f]Francesca[/name_f]!! I love [name_f]Sylvie[/name_f]!! I think it sounds fine with your last name. [name_u]Piper[/name_u] would be my second preference
I’m going with [name_f]Rhea[/name_f] [name_f]Frances[/name_f] - [name_f]Rhea[/name_f] goes great with her siblings names.
I don’t think that [name_f]Sylvie[/name_f] [name_u]Murphy[/name_u] sounds bad at all - I actually like the way it sounds together.
And don’t think it would be a problem with two S siblings- you do already have two T siblings 
You guys are awesome!! Thank you so much for all the feedback. It is so good to hear all of your opinions. I will have to sit on this for a while… but in the meantime, keep the suggestions coming!!
Oh, and a couple things… [name_f]Tessa[/name_f] and [name_u]Theo[/name_u] actually ARE twins. I know someone mentioned that along the way. And [name_f]Tessa[/name_f], who just turned 4, has a habit of calling her 2 year old sister [name_u]Sloane[/name_u], Sloanie (which is a nickname I HATE… but it is cute that she does it in the most endearing way). Hopefully this is something she outgrows, as Sloanie and [name_f]Sylvie[/name_f] are a little much.
I love the name [name_f]Sylvie[/name_f], but I agree that it doesn’t sound so great with your last name. I like [name_u]Piper[/name_u] as well, but my choice for you guys would be [name_f]Rhea[/name_f]… either [name_f]Rhea[/name_f] [name_f]Katherine[/name_f] or [name_f]Rhea[/name_f] [name_f]Camille[/name_f].
I think [name_f]Sylvie[/name_f] is lovely but I dont love it with murphy, and it may be a bit much with [name_u]Sloane[/name_u]. [name_u]Piper[/name_u] is nice but is starting to get pretty popular, if that bothers you? My pick wouldpbe [name_f]Rhea[/name_f], I think it is just gorgeous amd goes well with your other names. I like the soumd of [name_f]Rhea[/name_f] [name_u]Murphy[/name_u]
I’d rule out [name_f]Sylvie[/name_f], because there are too many things to take into account. [name_f]Sylvie[/name_f] [name_u]Murphy[/name_u] is a little sing songy to me, and classmates might think so too during roll call at school. Also you might not get great responses from friends if so many of them consider similar [name_f]Silvia[/name_f] a last name. And [name_f]Sylvie[/name_f] might make her a set with [name_u]Sloane[/name_u], in which case you have the T kids and the S kids, so to speak.
I choose [name_u]Piper[/name_u], because it sounds quite upbeat and current and I think that matches your other kids’ names. I think [name_f]Rhea[/name_f] sounds a bit heavy compared to the others. Also I [name_u]LOVE[/name_u] [name_u]Piper[/name_u] [name_f]Katherine[/name_f]:).
I agree that [name_f]Sylvie[/name_f] doesn’t sound well with [name_u]Murphy[/name_u].
Our surname also ends in a Y and has sadly cut out a lot of my favorites.
I think [name_f]Rhea[/name_f] [name_u]Murphy[/name_u] sounds the best.
[name_u]Piper[/name_u] [name_u]Murphy[/name_u] sounds fine too though.
So my pick would be: [name_f]Rhea[/name_f] [name_f]Katherine[/name_f] or [name_u]Piper[/name_u] [name_f]Katherine[/name_f]
I don’t think [name_f]Sylvie[/name_f] and [name_u]Sloane[/name_u] is a problem! [name_f]Sylvie[/name_f] [name_u]Murphy[/name_u] doesn’t flow great, but I like [name_f]Sylvie[/name_f] much more than [name_u]Piper[/name_u] and [name_f]Rhea[/name_f], and I don’t think it’s too bad!
What about [name_f]Sylvia[/name_f] with the nn [name_f]Sylvie[/name_f], to improve the flow?
I think [name_f]Sylvie[/name_f] [name_u]Murphy[/name_u] is fine. Not great but not so bad that I would think twice about it. If you’re at all worried about it I’d name her [name_f]Sylvia[/name_f] nn [name_f]Sylvie[/name_f]. I like [name_f]Rhea[/name_f] and [name_u]Piper[/name_u] too, but I like [name_f]Sylvie[/name_f] better. I also think any of the names work well with your current sib set.
There’s [name_u]Theo[/name_u] and [name_f]Tessa[/name_f], so [name_f]Sylvie[/name_f] and [name_u]Sloane[/name_u] shouldn’t be a problem.
I absolutely [name_u]LOVE[/name_u] [name_f]Sylvie[/name_f] [name_f]Frances[/name_f] [name_u]Murphy[/name_u], and the other [name_f]Sylvie[/name_f] names, but this one the most.