Missing link in family tree

Do you let your sims age normally, or on your own timeline? Is it possible the daughter’s age has been allowed to catch up with the mother’s? They can’t have the mother-daughter relationship if they’re the same age.

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.

Thank you very much, I will try that! Hope it will works!

PipMenace, the MCCC allows me to not age my Sims. The mother is an adult, and the daughter a young adult. But I think I understand the problem. I set up their relationship after creating the mom (I know, it's weird, but it works with her son and her parents, so I don't know why it doesn't with her daughter)

Ah sorry, perhaps that is or was a bug in my own game then, that I kept losing relationships when the ages overlapped. It’s been a while since I let them do so.

Nana_FE_2004​ Hope it all works out - let us know if you have any further issues with the relationship!

Update a few days later: I still have the same issue. And if I try editing the relationship with the CAS, I can only edit their relationship from "member of the household" to "partners" 

If you look at the daughter’s family tree, is there space for a parent or do they already have two (even if one or both is anonymous/unknown)?

Nana_FE_2004​  When you don't get the relationship choice that you want the sims to have in CAS, the only choice you really have left is to set the relationship through MCCC.  Just get both sims to the same lot and click on the MCCC option on either sim then select the Relationships menu option.  Choose the option to reset a relationship.  While it can be done using either sim, I find it easier and more straight forward to use the active sim.  I would also recommend that once you have the mother/daughter relationship reset, that you check the other household members to make sure sibling and extended relationships are correct.  Sometimes that doesn't happen and you have to reset the relationship bits for those relationships.

Hope this helps. 

On the daughter's familly tree, I don't see her parents. But they somehow appear as anonymus/unknown when I see her children family trees (which is really weird)

Okay, I will try that later today. Hope it will works.