Residential property which is the subject of a land trust may not be sold under a real estate installment contract unless at the time of execution of such a contract a full disclosure of the name of the trustee and the designation of the trust and all of the beneficiaries of the trust is made by the seller to the contract purchaser and the contract is signed by the beneficiaries having the power of direction. Each such “real estate installment contract” shall be deemed to include a provision, whether actually incorporated in the “real estate installment contract” or not, that the beneficiaries undertake to convey or cause to be conveyed the real property which is the subject of the “real estate installment contract” in accordance with the terms of the “real estate installment contract”. Any violation of this Act renders such a real estate installment contract voidable at the option of the purchaser.