Hi, I'd like to help you with your problem.
It sounds like from what you said that your daughter in law is distressed and perceives that her husband, your son, does not love her as much as he loves their daughter. You feel that your daughter in law is making up disorders and saying that your grandchild has them.
It appears that your daughter in law may be doing two things. She is saying that your granddaughter has these disorders so she may seem the hero and be there to "help" your granddaughter with illnesses that other family members cannot. Also, your daughter in law may think that these problems make her daughter less the focus of her husband and more connected to her as a result. The attention she gets from her daughter being sick may be the intention here.
Have you spoken to your son about your feelings and suspicions? This would be a very important first step. Bring your feelings to him in a nonjudgmental way, as more of a concern and willingness to help rather than accusing. This will help him be more open to your feelings.
Is your daughter in law harming your grandchild in any way? Is your grandchild getting sick more than usual and does it always seem a crisis? Is she hospitalized frequently? If your son is unresponsive to your worries, you may need to contact child protective services and ask what you can do and they can do to help the situation. Depending on the level of abuse by your daughter in law, this can be a life threatening situation for your grandchild. You can determine this by talking with the child protective services workers.
Also, recommend to your son and daughter in law that they see a counselor about this issue. Obviously, there is something going on in either the marriage or with your daughter in law that needs addressed immediately. They can ask their doctors for referrals or the local community mental health center may also be helpful.
It is wonderful that your granddaughter has such caring people as you in her life watching out for her. Keep in touch with her and let her know she can come to you with any concerns. Be cautious about coming between her and her parents, but provide a safe place for her in case she needs you.
I hope this has helped,