I think that Misty probably has good reasons for not having the girls buried and memorialized together.
One of which, in my opinion, is that little Lizzie would over-shine (so to speak) Lyric at the memorial. Lyric is smaller, cuter (in a traditional little kid way), and looks more innocent than her older, more developed cousin. Lyric looks less like a child, and therefore less innocent, cute, helpless etc (although we know she was all of these things because she's 10).
I was heavy and 5'6"ish at age 10 and had started to develop. Teachers treated me differently. I'd overhear them call the short, thin, not yet developing girls "so cute" and baby them. I felt like they and some adults thought I was an ugly duckling and would forget that I was the same age as those "little kids" despite being stuck in a woman's body (with size 11 feet). It was like they expected me to act older than the "little" girls my same age and be more mature/not kid around all cutesy like the "little" girls. I was taller than all but one of the boys in my grade & I always felt like some were suspicious that I and the other developing, larger girls would be interested in boys in a way that the little girls were not. Like developing early made us more prone to be promiscuous or something, when really, all we wanted to do was play Barbies and watch "The Lion King" like other 10 year olds & wish away all of the changes that were happening to our bodies. Many adults seemed to have a presumption we were somehow less innocent or less like children because we were bigger.
My guess is "little"/younger Lizzie would have taken away some of the spotlight from Lyric, at least in a memorial service. Putting pics of the two side by side might cause some to question that perhaps the older, more developed cousin was talking to someone online, meeting a boy, etc that led to this event and poor little Lizzie got dragged into it & hurt also.
I hope this all makes sense. I am not saying anyone at WS has treated either girl differently or as less/more innocent or as less/more of a child. However, my guess (based on my own experience) is that many of the adults (non-family members) who interacted with Lyric treated her somewhat differently from other "little" less-developed kids her own age and that as her mom, Misty picked up on this & we're seeing it in her decision to have a separate service/burial.
I grew for about a year more, eventually reaching 5'8ish with size 11 feet (thank goodness my feet didn't grow anymore). I eventually lost a lot of weight and entered college as 5'8ish and 135lbs/size 4/6 or so (not what I weigh/wear anymore) and outgrew my ugly duckling, kid in a woman's body, awkward 10-13 year old overweight developing-early phase. Lyric reminds me of myself as far as her size at that age (granted I was taller as well)- I am sad that she will never get to enjoy her swan phase where the adults (and sometimes other kids) who treated you so differently during that awkward stage see you and are shocked at the beautiful swan you've become (and you can tell they feel bad for how they acted towards you years ago). Just another thing this perp stole from these girls.