Bastila got turned by Malak easily and was afraid to face Malak again after Revan redeemed her, so I doubt her will is really that strong.
Its not about will. Its about the fact that their relationship is more than just lust and desire. Revan turned her back to the light. They shared visions and emotions.
And even if it is non canon, if Bastila became Revans apprentice in the dark side ending she never tries to betray him and remains loyal unlike most sith. They didn't fall for each other just because they wanted to fall in love. Its because they shared a strong bond and couldn't resist it. I believe if it was anyone else they would have showed restraint.