Bumping again for interest. I've come back to this POD, as well and who Arthur and his sisters might marry, again. I've also thought of a few things.
1. If the Pope still appoints Stephen Langton (IOTL one of the factors which led to the creation of the Magna Carta), would Arthur be more willing to accept it than John? I suppose he might if he thinks accepting it would gain him some measure of Papal support.
For Arthur, potential candidates I could see are:
1. Eleanor/Sancha of Aragon, daughters of King Alfonso II of Aragon. (I read somewhere that Aragon was more aligned towards England and Castille to France, but I'm not sure if that's true for 1202)
2. Richeza of Denmark, youngest sister of King Knut VI and Valdemar II of Denmark.
3. Aleidis/Ada, daughter(/s) and heir(/s) of Dirk VII, Count of Holland.
4. Margaret, eldest daughter of King William of Scotland. Or would such a match be seen as 'low' or 'barbaric', the same way the marriage between King Henry I of England and Edith-Matilda of Scotland was seen?
5. Maria of Brabant, daughter of Duke Henry I of Brabant and IOTL the second wife of Holy Roman Emperor Otto IV. They were first betrothed in 1198, but then it was put aside when, in 1204, Otto went over to the Hohenstaufens and later married Philip of Swabia's daughter.
For Eleanor, I could see her being matched with:
1. Theobald VI of Blois, son of Louis of Blois.
2. Peter of Dreux, second son of the Count of Dreux, OTL Duke of Brittany via his marriage to Alix of Thouars.
3. Ferdinand of Castille, eldest son of Alfonso VIII and Eleanor's first cousin, if dispensation could be acquired (probably unlikely under Innocent III).
I also thought of Henry of Flanders as a possible match, but then I remembered he's off on crusade with his brother.
And as for Alix and Catherine of Thouars, Arthur's half-sisters, I could see:
1. King Alexander II of Scotland, if a match between Alexander and Philip Augustus' daughter, Marie (there was one IOTL), falls through.
2. A son of Louis VIII of France and Blanche of Castille.
3. Philip Hurepel, the son of Philip Augustus by his controversial third wife, Agnes of Merania.
Any thoughts?