It is very difficult for the child. Also are you talking about biological parents or after remarriage, the new step parent? Both are different situations.
If you are talking about a stepparent, you can never expect the stepparent to feel the same for the child as the biological parent. If this happens - wonderful! But if the stepparent rejects the child of his/her partner - it is a very tough situation and there is no one straight forward answer. Usually you should clarify the situation before you become a family and blend. While you cannot expect from the stepparent to love the child, you can expect to respect it.
A lot of talking, understanding and usually a third party, best a family therapist or psychologist should be involved. It takes time and patient - and a lot of effort. Blended families in general take time and patience - making it a working family does not happen over night. So don't expect miracles - plan at least 1 1/2 yrs to make things work smoothly.
If the biological parent rejects the child it is much more difficult. First thing would be to find out where the rejection comes from and why? Then involve some counseling, see if the situation can be resolved or at least eased. Nothing is worse for a child than being rejected by his/her how parent. It will be a life time issue for the mental well being of that child. If nothing helps....it might eventually be necessary to separate the child from that parent - even if it means divorce. Again each individual situation can be so different, that it is almost impossible to give a detailed advice.