Mormons, If one does not get married in a mormon temple, is it true that they will never be able to find their family in heaven?

I don't think that we know enough about the conditions in the next life to be able to answer that question definitively. I heard many years ago that a person in a higher kingdom could come to a lower kingdom to visit people, but that is highly speculative doctrine at best.

Can two people living in the same level of the same Kingdom live together and be together? We don't know. All we know is that we are told that without a valid sealing, there is no guarantee that any two people, let alone their family, can be together.

Mormons don't actually believe that. and a temple marriage does not guarantee that all their family will be in heaven either.

Since there's no such place as heaven that's not really a problem.

Ask yourself - would any kind, fair, loving, forgiving god do that?

nope.