Re: Relationships!
I'd like to see this as well. I just added some family members to cover 3 households and it bothers me that I couldn't add Aunt or Uncle to relations by marriage. I can have aunt/uncle/cousin by linking them but my little Sim, Sonny is not related to his mum's brother's wife Odette and his cousin André has no relationship with his dad's sister's husband Jae. I just treated my mum's brother in law the same as if we were blood related. It's the same as in laws for adults. My dad called my mum's brother in law, his brother in law too and everyone I know does the same in their families, but my Sims have no familial relationship with in laws.
It seems like such a simple thing to add, especially since we got fiancé added. I know the family tree part of it might be a bit more complex to add but still, seems doable.