If there is nothing dubious about the family gatherings, it is permissible for a woman to sit with her husband’s relatives, if she is wearing complete Shar`i Hijab. However, shaking hands with a non-Mahram man is prohibited.

Your mother’s cousin is not a Mahram for your mother, let alone for you. It is permissible for him to marry you, as you are not a Mahram for him.

A woman’s mahram is a person whom she is never permitted to marry because of their close blood relationship or because of breastfeeding or because they are related by marriage.

A man is permitted to marry his maternal uncle’s ex-wife after she has completed her `Iddah. However, it is strictly prohibited to influence her or cause her to seek a divorce in order to marry her. If one marries his uncle's ex-wife, her children (one’s cousins) must be treated with fairness and kindness, and the bonds of kinship should be upheld sincerely for the sake of Allah.