I agree it's so basic to add these things to the CAS options and it's a constant source of frustration. My Sims mostly have big extended families.
There is a workaround but being limited to 8 Sims in CAS can get in the way.
If I create a Sim and want to create Cousins, I add a parent with a sibling to get Aunt/Uncle. I then create their child to get cousin. I can delete the Aunt/Uncle later to make space. I can add the parent's parents to get Grandparents and so on. Once I exit CAS I split the family members into their own households. Luckily I can create a granny flat pretty quickly for the relatives and once I set them up I just leave them to it.
Another problem is that they don't start out with a high enough relationship now like they used to and I'm finding that relationships are deteriorating faster and are harder to increase since the GRO patch (I haven't added the pack yet). So families are drifting apart, even with daily phonecalls.
For my biggest family network it takes a fair bit of work adding family members to get parents, aunts, kids and in-laws and trying to keep them all in touch with each other and more importantly for them to know each other. I try and add kids to a household of a couple of generations so the kids actually know their grandparents but there's always a grandchild who has to introduce themselves. It's worse with a celebrity Sim too because they can't introduce themselves! Judith Ward has adopted grandkids that can't speak to her because she's too famous.
