Missing link in family tree

Nana_FE_2004​  The game does this on occasion for some players.  You might be able to fix this in CAS if you use the cas.fulleditmode cheat before entering CAS then resetting the family relationship.  But that way is really hit and miss.  Sometimes it works, sometimes it doesn't.  So, I find it best and easier to use the MCCC relationships menu option to reset relationships when they go off the rails like in your case.

PipMenace​ Sorry, but you're not quite right.  Even when a teen ages up to young adult, the relationship remains with both parent and child now young adult.  The relationship in the family tree will show correctly as well as the life stage labels on the sims in the family tree displaying the correct life stage, not disappear as has happened here..  A same life stage parent/child relationship is possible if the child grows up in game.  It might be unlikely if you let kids go through every day of all life stages.  But for those who use a cake to manually age up the kids before the end of the early life stages normally, you can easily get a kid up to young adult before the parents age to adult so you can end up with parents and kids both as young adults.  That said, if you have that happen and have what this OP has had happen and losing the relationship connection (i.e. mother and daughter are both in the same stage) as well, you can't correct it in CAS as CAS won't let you.   So, then you have to reset the age of one or the other so they are in different life stages (FWIW, I would age the mother to adult, particularly if she has any discovery traits), then reset the mother/daughter relationship, then age the mother back to young adult.  Now, I always use MCCC to reset relationships as I find it easier.  CAS is not involved at all, you just click on one of the affected sims in live mode, then use the MCCC Relationship menu option to change the relationship.  If you have grandparents, you'll need to reset the relationship bits for them too and sometimes other extended family members too.  The only drawback is when you have same sex parents.  MCCC will only let you set one mother/one father.  It will not let you set two mothers/fathers.  So, you either have to live with it or try to change those in CAS.  

Hope this helps.