CAS cousins, piblings & niblings not treated as family/can romance in some cases

I have encountered the issues (stated in the title) in my main save, and I was rather concerned since they were still considered relatives under the relationship panel before the 3rd Feb update. Hence, I have decided to remove all my mods and custom contents, and created a new save and family to see if the problem can be solved, just in case it's due to the old age of my main save file as well as the possibility that it's corrupted. I created a pair of sisters first, and each sister has a daughter, so basically the two daughters are cousins, and I removed the sister pairs immediately while keeping the cousins, and clicked play. However, the same issue still exists, as you can see in the cas.png, which shows that Arlene and Kathleen are cousins, but they arent recognised as such in the family tree.png as well as in the relationship.png. What's even worse is that they are able to be romantic to each other even though they are cousins under cas. Hence, I wonder if it is possible for you to fix this glitch, perhaps by fixing the family tree (connecting two sims in the family tree via more than one unknown potraits). 

I am aware that this is related to 'Legacy Family Trees: Tree collapses/Overlaps/Shows wrong connections' (https://forums.ea.com/idea/the-sims-4-bug-reports-en/legacy-family-trees-tree-collapsesoverlapsshows-wrong-connections/13154809), but I wasnt too sure if I should elaborate on my issues with the missing family relationship under that bug post, as it focuses on family trees affected by the latest update. Thus, would you mind merge it with the aforementioned bug post if you find it relevant and connected.

Mods/CC: Removed completely and tested without.

Steps to reproduce: Create a pair of siblings, and each sibling should have a child, and the children would automatically be considered cousins. After that, remove the pair of sibling, and click play. For piblings and niblings, it's similar to creating cousins but you remove one of the siblings (the one without children) instead, and only one sibling needs to have a child.

Sooooo I have decided to do an experiment with a new save, whether removing the sister pair or one of the sisters right away in CAS or taking them into live mode and go back to the cas to delete them will impact the outcome.

Here's the results:

For cousin relationship: The outcome is the same when I deleted the sister pair, whether I created and deleted them right away or took both sisters to live mode and went back to the cas to delete them. Basically, the cousin relationship is gone from the family tree and the relationship panel, other than cas, and the cousins are able to be romantic to each other. Moreover, the outcome is still the same when I created and deleted one of the sisters right away in CAS. Surprisingly, when i deleted one of the sisters after taking the sisters to live mode and go back to the cas to delete her, the cousins still remain cousins and were recognised as such by the CAS, relationship panel and family tree.

For pibling-nibling relationship: The outcome is drastically different. The relationship was gone when I created and deleted one of the sisters right away in CAS, and hence the aunt and niece could be romantic to each other. In contrast, the relationship remained, and they are recognised as aunt and niece in the CAS, relationship panel and family tree.

Tldr: 

After the Feb 3 update, deleting Sims in CAS breaks extended-family relationships inconsistently. When parent siblings are removed, cousin and pibling–nibling links often disappear from the family tree and relationship panel even though CAS may still label them correctly, allowing romantic interactions between close relatives.

In a clean, mod-free test:

• If both parent siblings (the two sisters) are deleted, the cousin relationship is lost whether deletion happens immediately in CAS or after entering Live Mode.
• If one parent sibling is deleted in CAS, the cousin relationship is also lost.
• But if one parent sibling is deleted only after entering Live Mode and returning to CAS, the cousin relationship is preserved and shown correctly in CAS, the relationship panel, and the family tree.

For pibling–nibling:

• If the mother (who is the aunt’s sister) is deleted in CAS, the aunt/niece relationship disappears and they can be romantic.
• If the mother is deleted after entering Live Mode and then returning to CAS, the pibling–nibling relationship is preserved.

Overall, CAS deletion does not reliably maintain extended family links, causing the family tree and incest-prevention checks to desync.

UPDATE 1: The outcome is the same even after I used Clean User Folder Approach. By the way, I decided to do another experiment on cousin relationship due to the outcome of cousin relationship after i deleted one of the sisters after taking the sisters to live mode and go back to the cas to delete her. So, this time, I only went back to the cas and deleted one of the sisters after taking both sisters to live mode. After that I returned to the live mode, I went back to cas again to delete the other sister, and miraculously the cousin relationship remained and were recognised as such by the CAS, relationship panel and family tree. Hence, I hope this glitch will be fixed soon since the nephew in my main save is already an elder (aunt is also an elder) and it's not like I can just create him a mother, and then re-create the sister relationship with his aunt (between nephew's mother and nephew's aunt).

UPDATE 2: The same issue still exists after the Feb 12 update. Hopefully it will get fixed soon, so I will be able to go on with my main save. I really dont want to abandon it.

Edit By crinrict: Adjusted Title