Is eating food bought with a violinist's money unlawful (haram)? And is accepting his gift unlawful, especially if not eating leads to a severing of family ties between two sisters?

It is permissible to accept a relative's invitation, eat from their table, and accept their gift. Indeed, it is encouraged by the general prophetic narrations that urge accepting invitations and gifts. A condition for the permissibility of attending is that there should be no reprehensible act present, or that it is possible to denounce and change it.

As for shunning and boycotting people of فسق (disobedience/immorality), there is a detailed explanation: If there is a benefit in shunning, such as their return and repentance, then they should be shunned. However, if shunning only leads to estrangement and alienation, and it is for a sin that is not disbelief (kufr), then it is not permissible.