Islam enacts that the (female) cousin is a stranger (non-mahram ) to her (male) cousin, i.e. non-mahram. She has to wear her hijab in front of him. Aside this, she should also not be in seclusion with him. The Prophet (peace and blessings be upon him) strongly warned concerning the in-laws, since people sometimes mix freely with them without any limits which may cause severe problems in families.
He (peace be upon him) said,“Beware of entering upon women.” They said, “O Messenger of Allah, what do you think about the in-law [meaning the husband’s relatives]?” He said, “In-laws are akin to death!” (Sahih al-Bukhari)
Therefore, we should be cautious of how we relate with the non-mahrams who are basically the people whom we are not allowed to marry at all in any circumstance. We should should guard against all the doors that lead to corruption, the Prophet (peace and blessings be upon him) said, “Thedevil runs through the blood veins of the children of Adam!” (Sahih al-Bukhari)
We know no basis for her interference with yours and your husband’s affairs. It so odd, especially for she is married and has her husband as well whom she is required to pay her utmost attention to and care for him. Only, if she finds you in a sin or that you seek her counsel, in such case she can give you her advice in a complete decent and appropriate way.
Finally, we recommend you to care for your husband and attend to what he likes and looks for. The good wife is that which pa
ys her main and utmost care for her husband. Ahmad and Al-Hakim narrated from Al-Husain ibn Muhsin that his paternal aunt came to the Prophet (peace and blessings of Allah be upon him) for something and he dealt with her need, then the Prophet said: “Do you have a husband?” She said, “Yes.” He said: “How are you with him?” She said, “I do not neglect any of his rights except those I am unable to fulfill.” He said: “Look at how you are with him, for he is your paradise and your hell” – i.e., he is the cause of you entering Paradise if you fulfill his rights and the cause of your entering Hell if you fall short in that.
May Allah guide all to what pleases Him!