Help needed for a difficult scenario!

I would appreciate help to understand how to appropriately analyze the following data.

I have 3 groups: n1= 105, n2= 105, n3=15 (yes, just 15)

Groups n1 and n2 are related (i.e siblings), thus not independent of each other as the data are under genetic influence; group n3 is not related to either

Generally, I do not deal with siblings and for these type of data I use repeated measures ANOVA, but because of the different n sizes I have analyzed so far using non-parametric tests (Kruskal-Wallis and Mann-Whitney U). However, I am unsure this is correct because:

1. I do not take into account that n1 and n2 are related
2. perhaps some factor I have not yet taken into account?

What can I do to the data to take into account that just two of the groups are not related?
What is the correct analysis/test?
And how can I make a statement about power??

I would be grateful for any help!
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