Which minerall though? (there are two different kinds, one is Indoor - WITH vit.D, one is called Outdoor, no vit D)
Normally people use a calcium Without vit.D daily or most often, then occasionally use one with D3 (if its repcal brand, it's stronger so twice a month with minerall it's weaker so more frequently) and occasionally use a multivitamin like herptivite or reptivite a couple times a month as well.
So basically, if you've only been using minerall, it's possible they could be lacking in vit. A, for starters, and also depending on which minerall, could be either lacking or overdosing on vit. D as well.... I think both of these problems can be traced to tongue issues, as the two vitamins need to be in balance with each other.
As for the "young siblings" thing... The word young is a very relative term.
Normally by 3 months chameleons are shipped off to live on their own. I'm sure some are divided well before then also. You said 8 months?? That is plenty grown, and they should each have full claim to approx 2x2x4 cage territory by now.