Step I : FraticideEditNeither of the three kings has a son so if you manage to assassinate your brothers , you get their kingdoms. This might require saving and loading a few times but will essential triple your kingdom within day one. As Leon it is important to first assassinate the king of Castille so you get the money to kill your other brother and become heir to the kingdom of Galicia.
Step II : Appeasing the vassalsEdit
After uniting the three kingdoms you will need to give away some of the land and duchy titles to vassals so you won't have to deal with rebellions. If you do this right no one will revolt even if you passed low crown authority and maximum feudal levies.
Step III : SororicideEdit
Your sister , princess Urraca of Leon will certainly be a thorn in your side so either assassinate her from the start or hope your spymaster discovers a plot by her and you can imprison and execute her. If left alone she might band together with any disloyal vassals and press her claim on either kingdom.
Step IV : Parricide and MarriageEdit
Propose a marriage with Urraca of Aragaon the heir to the kingdom of Aragon and then assassinate her brother. With this step your heir will inherit the kingdom of Aragon.
Step V : The Reconquista starts at dawnEdit
A last tip is to conquer the emirates to the east first ( the ones that form the duchy of Aragon ) and Lleida.