I use the Custom Relationship label for this purpose. It means you can create a unique relationship between Sims or just label them as in laws for example. But I believe it requires the Lovestruck pack. I couldn't quite remember if it arrived with the prior patch (which would have been better IMO) or the pack because I added it on the day of release, I'm fairly certain it's part of the pack though.
I love this! I do this too with a mod I can't share here.
Edit: I don't think you need Lovestruck...just the right set of traits and likes that don't conflict.😉
