Relationships disappering
My problem is that the relationships just disappear over time.
My sims doesn´t know his dead wife anymore (their child can´t remember her either) and all the milestones (like loving her or marrying her) are gone too. Another sim he met disappeared too, even though he´s still alive and they met multiple times.
Please fix that. It´s sad enough that the child doesn´t know her mother, but the husband?
One thing you can do to keep relationships from decreasing is complete the Friend of the World Popularity aspiration. That earns the sim the Beloved trait. Once you have that trait, relationships will not slowly decrease over time. But you can still lose or gain relationship levels, both friendly and romantic, by how you interact with a sim. Sims will never be culled from your relationship panel either. The exception is ghosts.
As long as the ghost is still in the game, it will stay in the panel, but it will disappear at some point. If you have the ghost's tombstone/urn, that has an option to strengthen the connection to the world. Using that option will keep the ghost from being culled for a set amount of time, so you have to check the tombstone/urn every so often and keep using the option to keep the ghost in the game. If you use the Release to Netherworld option, then the ghost is immediately culled. I'm not sure how long the strengthen connection will be active for normal and short games as I play on long life. For long life, it looks like using the strengthen the connection option will keep the ghost around for around 90 days each time.
And if you want to keep the ghost forever without having to use the tombstone/urn option or if you don't have the tombstone/urn for the ghost, you can ask the ghost (if you can find them to talk to them) to join the household and become a playable sim again if they are at least a friend. This does take up a sim slot in the household though. I'm not sure it will work with cats, dogs or horses though as I haven't had an animal ghost yet.
There are a few ways to find a ghost. If you have the tombstone urn, you can summon them. Another way is to add the spooky lot trait to the home lot and a random ghost will visit your home every few nights. A third way is to go to a bar on Ghost Night on Wednesday night. That is probably the best way to find a particular ghost if you don't have the tombstone/urn since a bunch of them usually show up every time.
Hope this helps.