Is it normal to try anal sex with mother in law?

As I asked before about having threesome sex with my wife and mother in law, I want to share with you that we have started the threesome. My wife is very happy now that she can suck her mom's nipples, pussy and asshole. My mother in law loves getting fucked by me. We are really having awesome threesome.

But now we have another problem. My mother in law said that she wants to take my dick in her asshole. I never fucked my wife in her asshole and my mother in law is also an ass-virgin. Yesterday I tried to put my dick there in my MIL's ass, but I couldn't enter. My wife applied a lot of butter in her mom's hole, but still my 7" long and 5" round dick was unable to enter.

Now, guys, I know it takes time and patience to have anal. We have that. tell us more, how we can do this and make it more enjoyable?