When a Sim adopts, the child's name changes, and his family tree. So unless I make notes about all the orphans (and those removed by social worker) I won't remember their REAL relatives. Is there something that will stop his family tree from being wiped out? Or at least give me his original last name. So twins, separated at birth, can no longer fall in love...(I know I can set family relationship, but that doesn't fix the ability to fall in love,etc)

If you set a familial relationship with Found Family, it absolutely does make it impossible for them to fall in love. If you really want to preserve their original family ties and name, you can just teleport them to the lot directly from the adoption queue using a teleporter, but then they won't have any family ties with their adoptive parents and will be able to fall in love with their siblings, etc.

What is Found Family? Teleporting is a possibility, but finding the Sim to transport can be really time consuming; they are not always under their original family name.

Found Family: https://modthesims.info/d/620238/found-family.html

Teleporting is much easier if you go into SimPE and rename all the special families to something that makes sense.

Found family lets you set their adoptive family as family as well though, doesn't it?

Yes, you can set anyone as family that isn't already family.

CJH_NeighbourTeleporter.zip in orphan mode (Filter.../Orphans ON) only sims from the Orphan Family are available and your sim will adopt the neighbour teleported in. Parent/Child relationships are set but the family tree does not change. Once the adopted sim grows up to child or teen there is an option on it for the sim to move back to her/his birth mother's family provided at least one birth parent is still alive.

Interesting, what data does the family tree use to draw itself? I thought it relied on SRELs, but if you can change the parents without affecting the family tree that's obviously not the case.