Andromeda is an interesting case, as we hear a fair bit about her, but she only actually appears for about a page in DH.
We seem to generally agree she was a Slytherin, and the evidence overwhelmingly suggests it, especially the remark by Slughorn. I'm sure there are "nice Slytherins", but my inability to think of any off of the top of my head suggests that just being away from her family--just being at Hogwarts (surrounded by future Death Eaters and purebloods) --wouldn't have been enough to make her consider her parents' prejudices. It took falling in love with Ted."The whole Black family had been in my House, but Sirius ended up in Gryffindor! Shame."
-Professor Slughorn, HBP, pg 70
Should add--being Sirius' favorite cousin isn't saying much. From what we know, he didn't exactly have a lot of choices. Talk about sadistic psychopath and frigid snob.
I've never imagined Dromeda a rebel. Sirius' case was different--I've never found fanfictions in which Sirius gets emotional over the death of his father or Regulus' murder realistic. There is just a thing as digging too deep; Sirius pretty much genuinely hated his family. I don't think Andromeda hated her family. They must have been just as bad as Sirius', but there's never evidence she loathed them. She probably even loved them. She just loved Ted more.
Sirius ran away from something, Andromeda ran away to something.
In the end, Dromeda did it to be happy. I'm sure that sounds oversimplified to the point of being stupid, but it wasn't like all of a sudden she was anti-Voldemort and anti-Purebloodmania and wanted to fight or anything (Ted and Dromeda never actually joined the Order, did they?) It was Ted. Just Ted. And if the fact that he was a Muggleborn was just part of the package, take it or leave it. She was in love with him. She took it.
Concerning Remus and Tonks' relationship, Lupin's actual words were, "even her own family is disgusted by our marriage." I reckon there are two ways to see this: either she knew what it was like being married to someone shunned by high class society and didn't want that for her daughter, or she and Ted are prone to prejudice and hypocrisy just like everyone else. The flinch-and-recoil reflex is ingrained. For Tonks, Remus' being part of a werewolf was part of the package, take it or leave it. Sound familiar? Yet Ted and Dromeda's disapproval was strong enough to make Remus miserable for the first half of DH, on top of his own guilty conscience. Dromeda's character is far from perfect, even if she is loads of fun to read and write about. You just have to take it with a grain of salt.