I've had this happen to two of my sims and it's so frustrating. The first lived with Don Lothario, I was playing her brother's family for a while and when I went into household management, I noticed she was gone. I checked and she wasn't in the relationship panels of Don or her brother, she had just disappeared! And then last night, I decided to play Don for a while despite her disappearance and adopted a baby, then went out to the neighbourhood and noticed that the brother of the first sim to disappear was gone now too. He was gone from the relationship panels of his wife and kids too. I really hope this can be fixed soon!