Went on a vacation with the Sigworths and Sims seemed to go up stairs fine. Switched to Judith Ward and she won't go up stairs to save her life (or at least, her bladder). I'm playing with an install ...
Update; Turns out this is actually a railing issue. Removing the "fence" at the top of the stairs allowed them to go. I haven't played TS4 in a couple of years, but I'm assuming this is a new issue - don't remember these locations having this problem before.
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