The granting of planning permission does not override other legal constraints. You can apply for permission to demolish and replace your neighbour's house, but that does not give you the right to go ahead.
If the non-applicant sister was actually a part owner, then this should have been stated by the applicant, and the applicant should have given notice to the sister. The non-owner would be entitled to object to the application on valid planning grounds, but she couldn't demand that permission be withheld simply because she didn't want it.