It's a 3way relationship of course things are going to happen between the girls too.
It's not a matter of faithfulness, they will all be together. Sandra will be with Haley as much as she will the MC, that's how those relationships work.
Same with proper harem games. When the MC isn't fucking the girls they all fuck each other.
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.