Speaking as one who lived in southeast and southwest China for over 30 years.
爱人 can be applied to husband and wife alike, but it cannot be used in plural. The dictionary says clearly: 爱人:丈夫或妻子。 恋爱中男女的一方。(But practically we don't call one's girlfriend or boyfriend as 爱人, only husband or wife, unless you are being intentionally humorous or poetic )
Never heard of people calling their daughter-in-law directly as 媳妇, it has to be 儿媳妇, 孙媳妇,弟媳妇,etc.
Therefore, to me, 媳妇 is simply a dialectical alternative of wife. (Although the dictionary does say: 媳妇:儿子的妻子, it will be extremely weird for me to hear my father calling my wife as 媳妇)
Another thing is, people in a romantic relationship (especially when they are young and not married) is more referred to as 恋人. (it will be weird to call a married couple 恋人. )
情人 is now almost always used in an unethical context. The third wheel in a relationship/marriage, the person who you have an affair with.