BayesTwin is an R package which can be used to perform bayesian analysis of item-level twin data. Simulatenously with the biometric model (ACE/ADE or AE), an item response theory (IRT) model is estimated to take into account properties of the measurement scale. Functions are included in the package that help plot relevant information in figures and compute posterior statistics such as HPD intervals.
A manual of BayesTwin can be found here: Manual
Here you can find sample scripts: