I very much agree!
It seems like such a simple addition, im surprised it hasn't been patched in yet... In laws and step family are usually HUGE parts of families in real life, I think they should definatly not be ignored in the relationship panel. They updated it with parenthood, adding different friend relationships (I guess?), so it seems so weird that this is constantly so overlooked! .... And honestly if they weren't added then and we're 3 years in, im not holding my breath, but I'd seriously love it so much if they added new relationships.