His father-in-law was a slave owner. And his platform in the election of 1860 was not abolition, but rather just stopping the spread of slavery. Like he was grandfathering it in for some states. Now I’ve heard and read this wasn’t that he was tolerant of slavery but rather that he didn’t think the federal government was empowered with abolition.
But apparently he also had a good relationship with his father-in-law...