How to pass 1-4 unit investment properties to children

[Jon Q.]

How can someone avoid probate and the due on sale clauses when passing 1-4 unit investment properties to children?  What structure works?

I'm most interested in hearing from people who have actual experience and have been successful in accomplishing avoidance of both due on sale clauses and probate.

I have a relative with lots of 1-4 unit investment properties in several states who would like to avoid probate, taxes, and due on sale clauses and have the property passed to one of my siblings.

Thanks in advance for any responses.