Upon further investigation, this seems to be affecting one parent, but not the other.
the mother Sim was at home with the infants (twins) upon last save, and loaded into game this way post-update. father sim was at work
i hired a nanny, who was able to care for infants without issue. when father returned home, he was able to interact with infants without issue. only the mother sim has trouble. after introducing infants to mother using father as active sim, mother is able to interact, but does not have option to breastfeed infant, and pie menu is structured as if she is a visiting/non-family sim.
ne save file is now functioning as expected.
@CrzyCrt - This sounds like the original bug from Growing Together has resurfaced. Something in the latest patch seems to have undone a few older fixes from the reports I've seen.
