No, I do not believe love is unconditional, even in families.
To use a dramatic example, what of those family members who murder other family members? Do other family members still love the perpetrator? In cases where there are extenuating circumstances, such as murdering a child molester; perhaps. In others, where children kill their parents just to get the money - I'm thinking not so much.
In a marriage, I suggest there are, even if unspoken, expectations of behaviour. We expect our spouse to be faithful to us (unless the relationship is an "open" one). We expect our spouse not to commit crimes. If they transgress, the love we had for them dies in many cases.
While we may still love our children through such challenges as drug addiction or alcholism, other issues may well cause parents to disown the child emotionally.
I think unconditional love is a lovely concept, but in reality it does not exist.