Competition in Cable Act of 1991 - Amends the Clayton Act to prohibit any person: (1) engaged in the distribution of a cable programming service that is owned or controlled by, or affiliated through common ownership with, any one or more cable systems, from discriminating in the price, terms, conditions, or availability for sale or license of its programming among cable systems or other multichannel video programming distributors who purchase or license such programming for delivery to consumers; and (2) from acquiring an interest in any cable television system where such person owns, controls, or is affiliated through common ownership with, cable television systems that are available to more than 25 percent of all cable subscribers in the United States.