What do you do if your cousin's father denies that your grandmother breastfed him, asserting that she had no milk, and that the issue of breastfeeding was fabricated to prevent marriage between the two families?

The testimony of the grandmother and the maternal aunt does not establish suckling (رضاع) according to the majority of scholars. Those who affirm it through the testimony of one or two women stipulate conditions that may not be met by them. If suckling is not established, then marriage is permissible. However, if the purpose of raising the issue of suckling is to prevent the marriage, then there is no solution for that, as they can make excuses in other ways. It is recommended to perform Istikhara (prayer for guidance) and supplicate for the facilitation of good and the averting of evil.