One of the most difficult aspects of blending a family is that your values may be very different from those the child comes with, making liking or bonding difficult. If your dislike continues over time, again, try getting some professional help. If you continue to dislike the child, breathe deeply and do the best you can. Whatever you do though, don't let it become a thorn in your marriage. This is one time, slow and steady can win the race.
Boundaries are very important here.
Time with you is time with you. Time with them is time with them. You are the grown-up. You have chosen this package. The last thing you want to do is openly compete.