Hello! I'm looking for a mod that will allow me to edit Family Trees with my Sims. I created a family and then created a SIM who should be the spouse to two of the adults. Is there a way for me to set this family connect between the SIMS? They do live in the same household.

Use Woohooer to enable polyamory with no jealousy and there's a Registry Office mod that enables sims to marry multiple people at City Hall regardless of whether they live in the same household or not and also enables sims to divorce and remain friends.

It is my understanding that EA's game build does not allow two spouses. There are some work-arounds that might help you on the NRaas Relationship Page FAQ :
Change The Relationship Status Of Sims
Adding Or Removing A Parent, Child Or Sibling Within A Household
Multiple Spouses - Polygamy

Yeah, there are tricky mod related ways to allow for a sim to have more than one partner. A partner is defined as a steady BF/GF or higher. And there are some in-game ways this can happen -- Proms, some of the goofier Photo Booth interactions, I think elixirs or maybe it's witches' spells, the Relationship Transmogrophier that comes with Level 10 on the Social Networking skill. But none of those apparent multiple partnerships will be stable as the game relies upon at most one partner data field per sim per world for many of its operations.

I'm afraid for various other reasons we can't really recommend the Registry Office mod but good luck with it if you choose to try it out.

The game is just not set up for multiple partnerships within the same world. The only way that can really work in any ongoing sense is if the multiple partners live in different connected worlds (WA and/or NRaas Traveler mod) and are never in the same world as your sim at the same time. The best most of us can manage is to never let the multiple relationships go beyond Romantic Interests and distribute the No Jealousy LTR liberally as that just seems to be the path of (much) less resistance.