can we attend weddings where there is free mixing but we know all the people, like relatives and cousins and families are all known or is it not permissible? and if not , by not attending are we sinning cause den the people that have invited us wont be happy with us

Mixing at weddings is not allowed, even among relatives and cousins. If your attending will stop the mixing, attend, and if it will not, do not attend, and you are not sinning by that. The scholars have said that if they cut you off for it, they are the ones sinning.