ANSWER:

You are not obliged to entertain his mother or obey him in this.

Simply be polite to her in the best way you can and call your mother on the phone in order to check up on her often without telling your husband. 

My husband constantly wants me to spend time with his mother who is easily bored. I try from my end but my MIL is difficult and it is straining my marriage. She is always unhappy and complaining. He doesn’t let me visit my mother much as he wants me to spend time with his mother & doesn’t like my family. I’m always caught in the middle of a blame game. I don’t know how to keep her happy & busy.