Apparently the weirdness with Alexandra and Carlo was just the beginning. The hood has gone completely bonkers and I haven't been able to fix it even though I've tried every possible method I can think of.
5 sims in 3 separate households have become invisible (Erin & Regan Fielding, Ben Cooper and Jack & Olivia Robinson). Their icons are still in the UI with other family members but you can't direct them to do anything and the game crashes if I try ie. change their appearance via SimBlender. I moved the uni graduates into those households so maybe they are the ones causing the problems although they seem to be fine. But then again one household (Sam Cooper & Grace Robinson) has been unaffected by this "plague" so I don't really know what's causing it. I don't feel like killing off 5 sims especially since I don't know if that'll fix the problem.
It looks like Wellington has become a BFBVFS and this might very well be the end :(
edit: Clearly I'm not the sharpest pen in the box... I remembered I made a backup of my wh9ole TS2 folder before I re-installed my game on Saturday. I used the backup, played the girls through college and this time everything turned out fine. I'm back in business!!!