This has been happing to me for a long time as well. It happens with all households. When I select the travel option on the phone, it will only let me select one additional sim to invite. If you click on another, it has the same reaction as if you were trying to invite more than the maximum number of sims allowed. It does not matter if the sims you are inviting live in your household or not.
I am not sure if a specific action is triggering this, as it seems random, but it will last for the duration of the time the game is running. Quitting and restarting the game seems to fix it. This bug happens maybe 25% of the time.
