Hello there I am needing help on something.
All of the expansions had made my game pretty hard to keep up with, so I am thinking about moving out a sim´s brother. I know how sims can keep contact with each other, but I am wondering about the family pet. He currently has 100´s mutual with the kitty and I hate tearing up a family. Is there any way he can keep contact with the family cat?
Thank you!

Teleport it over to his house for visits.

Are you sure pet repationships even decline?

Use the "invite household" feature to invite the kitty over for Sunday dinner along with the rest of the family. Keep your eyes open and rush out to greet everybody any time a family member walks the cat past the house. Invite the kitty to parties! (Seriously, you can do that.)

They can decline. And yes, weirdly, you can invite pets to parties and also bring them on dates and outings!

You could also invite the brother over to the house where the pet lives and he should autonomously interact.

Thank you so much everyone! The cat will have her best friend back!

Don't forget to tick all the perks related to needs, this makes the sims easier to look after than Ep's before this. Having more EP's should not be making things harder.
If you invite pets over make sure to have things for them to keep their needs up too. If Kitty has no scratching post available she might decide to use his furniture.

You do need to make sure you have pet items around the house before inviting pets over; but as a former Sims1 player (when we had to fill all needs uphill both ways in the snow) I really can't see ticking all the need-related perks. That would make it way too easy and pointless for me! The lower the fun, the longer they'll stick to a fun-giving task; the lower the social, the more they'll autonomously seek social interactions.

To each her own.

I'm with you Peni. It irks me that I sometimes have to fill a need-related perk to unlock other perks. I'd happily make all needs except energy drop faster, if there was such a mod.