You can be with Sandra....I dont see the point. To tell Haley's story "we" have to date her? Thats is really necessary? The way i see, her story can be told even if Haley and the MC are just brother and sister. If the MC marries Sandra, haley's story still can be told, this would only change a part of her story, but still would be her story.
I dont play the game anymore (after the last update) and i never liked Sandra, but i dont see why the MC cannot be with her if the person playing likes her more. MC and Sandra like a couple could help Haley to overcome her problems...
The people telling you that you can't are straight up lying. If you had played this update you'd have noticed that what you're stating is actually happening.
But you know complaining without playing and just taking butthurt peoples words as gospel is the way to go.