I get that you like this development and I get that you like Sandra more. I also agree with your earlier assessment that Sandra is a "better" person then Haley. Haley is manipulative and Sandra is sweet, naïve with a bit of OCD. Therefore I understand your choice with the threesome.
For my assessment, Sandra is just a tool for Haley to make her relationship with her brother work. She does not love Sandra. She is using Sandra. She pushed Sandra and her brother into a relationship. The brother may or may not have fallen in love with Sandra, but It's kind of the only reason Haley decided to let Sandra into her plan.
Normally I would called Haley a manipulative b#!c, but she can not help herself. She will do anything to keep her brother, including keeping Sandra around. I felt that it was wrong to drag Sandra unknowingly into this relationship. I think Haley can not truly love anyone besides the MC.
It's not like Izzy, Alexandra and puppy. They are truly in a 3 way relationship. Izzy and Alexandra loved each other way before the Pupster walked into their lives.
Not criticizing, just sharing a different perspective.