hi cass. i have a similar situation.
i think its only natural to resent the things that having step children can cause.especially if it wasn't totally your decision like that! but its important to also appreciate the good things they bring.
i sometimes wonder if my first child (and second) would have more attention from her father if she wasn't his 3rd...but his first (and second) aswell. her life would be different somehow yk?
but it is wonderful for them to have elder siblings who play with her and adore her. they really do get the best of both worlds.
like everything in life, its all about give and take