I looked up ISELA in Seibicke's HDV*, because I vaguely remembered having read it before. No Isela, but ISELIN. This is a name of Germanic origin, containing the element ISEN, iron. I can well imagine Iselin being changed to Isela in Spain or Italy (like Ferdinand to Fernando), but I have no proof for this.
* Historisches Deutsches Vornamenbuch
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