I had a great relationship with my children's step dad.....he was wonderful with them and still is, even though we aren't together.
The key, I think, to not being seen as the wicked step mother is down to the man. THEY need to be the ones disciplining their child, THEY need to be the ones making sure children are treated fairly, while also giving their own children time so they don't feel neglected. So many men only see their children at weekends, and their time is so precious with them that they don't want to tell their children off. But will happily tell off their step children that they live with (they of course should be able to do that!). But there can't be two sets of rules.
I remember my ex step daughter once tried to push her brother down the stairs when they were here. All she got told by her dad was "awww that's not very nice is it". Yet mine would talk at dinner and get told off!
Or they would be spiteful to my kids (my step daughter once pulled all my daughters clothes off the hangers because she 'took up too much room'). Her dad did nothing, so I had a word with her. Of course, I then became the wicked step mother!
I recently had a 6 month relationship and part of the reason I ended it was because of how his 6 year old daughter was. Full on spiteful, told my kids to shut up and called them stupid (all older than her too!) And when they told him, he did nothing.
I also saw her attempt to hit her dad over the head with her kindle, so aggressively that he cowered. He said nothing! I told her we don't have hitting in my house!!!!!