Equating means and variances across MZs, DZs and Siblings

Hi everyone!
I have a question about equating means and variances. I am working with a small twin data set (about 400 MZ, 700 DZ and 400 siblings - individuals not twin pairs). I am trying to run some simple univariate analyses for a couple of different variables, but for some of the variables, I can see in the assumption checking that the fit gets worse if I try to equate the variances for MZs, DZs and siblings (means are ok to eqaute). Even if I leave out (or not equate) the siblings and just try to eqaute the variance for the MZs and the DZs, the fit still gets worse. What can I do? Thank you very much for your help!