Relationships Should Be Easier to Modify

My biggest headache in the game is trying to modify relationships. I really wish it were easier. I downloaded some cool Sims and tried to set Straud as their parent, but the game only gives me the option to make him their brother, husband, or roommate, even when I change his age. There is some glitch every single time I try to modify relationships in this game. For me, relationships are a big part of the game, so this really drives me crazy. I have literally spent hours trying to modify a relationship in the past. I would love to see this fixed!

Edit: It was a lot easier this time because I took notes the last time I had this problem. None of the usual relationship cheats work for me.

Here are the steps that work, in case you have the same problem:
1. Save one Sim to gallery. Delete the Sim from the first game.
2. Create a new save file and download the Sim into the new save file.
3. Upload the Sim to the gallery again.
4. Return to the original save file. Download the second save of the Sim back into the original save file.
5. Change the parent Sim's age to elder and the downloaded Sim's age to young adult or younger.
6. Change their relationship. It should now work!
7. Change their ages back to the desired ages. However, they can't both be young adults, even if they are vampirically eternal young adults. They can, however, be an adult and a young adult.

Second Edit:
About a year ago, I created a household with my dogs as my Simself's human children. One of my dogs is old, so I somehow managed to make him elderly AND be my son. I didn't take notes, so I'm not sure how I did it. Therefore, it's possible, but it probably took me at least an hour of trying to trick the game.