Am I obligated to obey my husband and wear the hijab in front of my physically and mentally disabled maternal cousin, who behaves like a seven-year-old child?

You must veil yourself from your maternal aunt's son if he is discerning and close to puberty. However, if he is not discerning, or if he is discerning but has not yet reached adolescence, you are not required to veil yourself from him, unless your husband commands you to do so, because obedience to him is obligatory as long as it does not involve disobedience to Allah. The fact that the maternal aunt's son is physically or mentally disabled does not affect this ruling, as long as he desires women.