I'm hoping for some advice. I have a longitudinal dataset with two variables, measured 3-5 times per individual (so 3-5 timepoints per subject). Both variables are measured on the same day, but the time between timepoints is variable between individuals, and between visits within an individual.
This dataset also contains related samples (twins), so there is family structure in addition to the multiple measurements per individual.
We are interested in assessing how the relationship between the two variables changes over time.
I would be very grateful for advice on models that could address this question. I am an R user, and from my reading I have come across suggestions of autoregressive linear mixed effects models with visit as a random effect, but am unsure how exactly to code such a model.
I would really appreciate any advice you could offer on appropriate models.
Thank you in advance!