Is it permissible to inform a paternal cousin that his wife is a liar and that his brothers are truthful, with the aim of guiding him to what is right and saving his daughter and his relationship with his brothers, or would that be considered spoiling his marital life and meddling?

If the wife lies to her husband and falsely accuses his brothers, then it is obligatory to advise her by explaining that lying and falsely accusing people are forbidden, and that if it leads to severing ties of kinship and corrupting relations, it is a heinous major sin; because the Prophet, peace and blessings be upon him, said: "Beware of evil relations among yourselves, for it is the shaver." If she does not heed the advice, then you must clarify the truth to her husband and repel the false accusations against his brothers. This is part of advising Muslims, changing evil, and striving for reform.