In most cases, this is done through mind control manipulation to create complacency and to redirect thoughts if any questions or doubts arise about how the person might be behaving. Certainly, in a situation with a world leader gaining power there would be many reasons to dismiss any change in the person by friends and family as being attributable to the greater stress and the need to focus more broadly, and in some new ways, to serve the nation or other institution being under their charge, and to grow beyond what they were as a prior independent individual without such responsibilities.
If needed, the family members can be replaced as well. This will particularly be true if there is a close working association and the need is present to have them agree with more negative choices that will affect a broad array of ongoing operations or numbers of people. This will be done to avoid inner circle discord but will depend on how they can be managed without having to go to this length.