Like in Spaceballs the son-in-law's fathers brothers mothers uncle makes us absolutely nothing. A/Z is limited to direct familial relationships. So his daughter and son-in-law can use Z plan, but he can't because he is just a friend. Z also excludes cousins.
From the form:
Relationship to Sponsor*: (check one)
Self | Spouse | Parent | Grandparent | Brother/Sister | Son/Daughter
Grandchild | Same-Sex Domestic Partner | Aunt, Uncle (A/Z-Plan only) | Niece/Nephew (A/Z-Plan Only)
Friend/Neighbor (X-Plan Friends & Neighbors only) | Household Member (X-Plan Partner only)
NOTE: Cousins, Friends, Neighbors, and other Household Members Are Not Eligible for A, D or Z-Plan. Aunts, Uncles, Nieces, and Nephews are not eligible for D-Plan.
* Ford Partner Recognition Program participants may only sponsor other people living in their household (proof required: customer must have same address as sponsor on their driver's license). Non-eligible members may appear as co-signers or co-makers of the loan or other financing agreement, and may also appear on the Vehicle Title or Registration. However, the primary user of the vehicle must be the AXZD-Plan eligible member.