Per this discussion post: do siblings who look alike tend to get along better and help each other more?

Ideally you'd get a bunch of (full, tested genetically if there's budget and this passes the ethics board) sibling pairs, probably controlling for number and variety of extra siblings in the family. Take pictures of all the pairs, get third parties to rate their visual similarity (I can name half a dozen features that don't match between me and my sister but that didn't stop six of her friends from mistaking me for her when I walked into her school building one day, so I'm obviously no judge of the matter), and then measure altruism between the siblings (exchanges of babysitting services, frequency of contact/social support, any material assistance that they provide one another, etc.)