It isn't an awful thing to say. It is a realistic thing to say. There is almost nothing that you can do. It seems there is little that dhcan or will do with/about cgfg. He may not even see a problem with her getting preg at 13. Until the situation comes up you never know how another person will respond. I hope he would be horrified and want to get her on some form of birth control like norplant or the shot, but you never know. We had neighbors who were thrilled when their 15yo started planning her wedding for right before her 16th birthday. I have known people who thought that there was nothing wrong with a young teen getting pregnant as long as she knew "pretty much" who the father was. Yes, I said "pretty much", which is exactly the wording the parents used! In some rural areas these very early marriages are more common than you might think.
It might be worth it to call husband's divorce attorney. One of my aunt M's prior husband's, C, had a very messed up family. He wound up paying child support for a spoiled girl who wouldn't even speak to him if she ran into him on the street until she was 21. By that time she and her boyfriend and their 3 kids all lived with her mom because if she moved out or got married then the support would stop. They even tried to get the support extended to age 24 because she couldn't go to work or school since she had 3 small children.
Make sure you know what the divorce agreement says, and ask the attorney what you can expect if she gets pregnant.
It is a dang shame that her mother is such a sorry parent. Her refusal to work with you and husband on ANY matter concerning cgfg is a big contributing factor to this. Having a child as a teen generally means some pretty miserable things for your future. It doesn't guarantee those things, but it makes them more likely. Teen bodies just are not designed to carry babies, esp at that age.
Do NOT be shocked if you end up taking her to the doctor and the doctor won't do anything. I took Wiz at 14 because we learned about something. The pediatrician read him the riot act and flat out refused to discuss any birth control except abstinence. We were lucky because the girl he was interested in lived over an hour away - they met in the acute psychiatric hospital (his second trip to the acute psychiatric hospital was to see her, she emailed that she was going back there - grrrr). He didn't have an interest in any local girls for a few years after that. I doubt you will be so lucky. Hopefully with a female child the doctor would be more realistic, esp given the damage that pregnancy at her age could do to her body.
Chances are that if gma won't work with you, and husband won't work to really force the issue (which would likely backfire, given the mom's stupidity), all you can do is to keep the condom supply stocked and pray that if she does need them then she knows how to use them properly. And that she doesn't need them.
Has she ever heard about the ugly, painful, unpleasant parts of pregnancy? Morning sickness all day long for 9 full months, hemorrhoids (show her some pictures of them online), swollen feet and legs that feel like they would pop if you sstuck a pin in them, headaches, backaches, having to go to the bathroom as soon as you start to do anything, having the baby get up under your ribs and try to kick them out of your body, etc... I let one of my younger cousins hear me gritch about the yucky parts while I was preg with thank you - it helped her slow things down with her boyfriend pretty substantially. A few years ago she said that some of the things really grossed her out - esp the part about the pelvic exams every couple of months and cervix checks during labor. Just the idea of some man or woman you don't know sticking a hand in there seemed grosser to her than anything she had ever heard, lol.
I am sorry that the entire family (well, adults and cgfg) has to worry and wonder about this.