Yeah, it can be hard - and there is THREE of them! don't be too hard on yourself.
If you can pull yourself back a little bit and see yourself as the aunty (or something) it can change your perspecctive.
i think the biggest mistake we all make is feeling like you have to be a mother to them. You don't - they already have one, even if she is a bit crap.
I found it really hard, esp when they are young but now my step kids (ex to be technical but I still love 'em) are older it is such a joy to be part of their lives. It adds I dimesnion of step parenting I didn't expect.

good Luck with it!