The kingdom of Aragon, a one province minor county at the top of iberia, bordered by the duchy of Barcelona to the east, the kingdom of Navarre to the west, and hudid emirate to the south.
Led by King Sancho, 6th in line to inherit the 3 Iberian kingdoms, with a claim on the kingdom of Navarre, is perhaps the most IDEAL and deserving candidate to unite all the spainish thrones.
Invite your youngest unmarried sister to your court
Marry one of your unmaried sisters to the heir of Barcelona, for an alliance and to befriend the current Duke of Barcelona
marry your other sister to the current King of France, or to the current Duke of Toulouse, securing another alliance and security.
break your current betrothal and marry someone with better traits if you want
declare de djure duchy war against the hudid emirate to your South, calling in both alliances
win the war, securing them as a vassal
join the Duke of Barcelona in their dejure duchy war if the start one, thus befriending the Duke.
Wait for the inevitable vassal revolt, calling in your 2 alliances as defenders for free
win the war, imprisoning the revolter
increase crown authority and revoke everything from him, securing 2-3 counties for yourself
revoke the last 1-2 vassals and call in your allies when they revolt. If your alies have them imprisoned already it will be an instant 100% warscore
revoke their counties
revoke all the infidel barronies, and possibly your brother who may be an infidel barron too, who you can then marry off to someone for traits
grant revoked barronies to a local (Christian) noble
improve relations with the Duke of Barcelona and peacefully diplomatically vassalize them, since you are the king of Aragon and they fall under your dejure lands.
Now, what you must do if you feel inclined is to start murdering the claimants and kings of the 3 iberian kingdoms, including any of their children until you are in line to inherit their thrones. Successive murders will make it easy since you will have many agents who hate their new ruler. If you are lucky, the bubonic plauge may decimate many of them for you.
Once you have inherited the 3 kingdoms, clean up your realm by creating duchies and granting them to decent people of your choosing, and place the rest of your unruly county vassals under them, thus securing your realm internally. Also grant powerful vassals a position on your council and sway them if need be.
Next would be to push your claim on the kingdom of navarre and turn him into your vassal, who is also a friend to you and will not cause any problems. Now, with all the Christian kingdoms under your control you can take the decision to unite them all into a single kingdom of A R A G O N spanning from Santiago across to Barcelona.
To aid in your holy wars against the infidels, you can bring the large duchy of aquitaine into your alliance ring, completely securing your northern borders and bringing another crusader state into the reconquista.
Historically, the reconquista took several centuries so don’t feel like you need to rush it. You can spend your time breeding your grandchildren to talent or with each other to improve their traits. To do this, don’t grant you children land until the later stages if your life, because if you do, you won’t be able to control who your grandchildren marry.