The only way someone else can negatively influence what you want to manifest is if you give your attention to what they are doing, and in doing so, influence yourself negatively through your observation of what they are doing.

If someone else is in disagreement with what you want then the simple solution is not to share with them or discuss the details of what you want. Just keep it to yourself...they are just thoughts, after all. And you are entitled to have private thoughts, aren't you? :)

If you don't keep it to yourself, you then risk the other person pushing against what you want and then you may find yourself instinctively pushing back against the other person's pushing. In that case, everyone is out of alignment with what they really want - they all end up focused on depriving the other person of what they want instead. And then all that happens is that no-one gets what they want and then everyone blames each other for the outcome.

Just keeping quiet for a while seems like a safer option :)

And anyway, part of your desire for what you want will probably be for you to get what you want in a way that the other person is likely to be happy as well, won't it? So getting what you want (even if you have to manifest it secretly) will probably be good for both of you anyway.

The Universe is abundant and resourceful enough to satisfy the desires of all the aligned parties involved in any relationship through a blended manifestation provided they don't use each other as excuses to deprive themselves.

This answer about conflicting desires explains the idea a bit more.