Thanks for your question. I will try to help you.
Unfortunately, your neighbour can put up a fence with a gate. He can also go a step further and put a lock on the gate provided that he provides your sister with a key, which he is proposing to do. As long as your sister has a key and can open the gate then this is not classed as an obstruction.
Harassment is subjective. It is down to your sister if she considers that her neighbour is engaging in unwanted conduct towards her. The conduct can be physical, verbal, non-verbal. TheRe does mot have to be motive or intention, so conduct can be classified harassment even if her neighbour does not know there is harm caused by his actions.
Has there been any incidents which cause fear of violence? This must happen on at least 2 occasions.
Harassment is a police matter. You can speak to them to establish whether the effect of the neighbour's actions can be dealt with by them
I hope this is helpful.
Do you have any queries or questions?