My daughter in law is a control freak. Her mother is exactly the same. My daughter in law seems to delight in fighting with me in order to keep me from seeing my granddaughters. My son is easy going and to keep the peace goes along with her behaviour. Ever since she came into the family she has caused trouble by her insecurity and wanting to know at all times where my son is. They have 2 daughters, youngest born in Jan 2010 and I have seen her one time and that's it. My daughter in law caused another "fight" so I am not welcome in their home according to her. My son promised to bring the girls to my house as he said he did not want his girls growing up not knowing their grandmother. Since then I have rung every week until recently and he very occasionally rings me. I speak on the phone to my older granddaughter who is nearly 4 now but that's it. When the second baby was born it was 2-3 weeks before I saw her and we went to their house and my daughter in law left and went to her mothers place. She then rang and asked my son if we had gone yet. She is impossible to get on with, she rules the roost which is my sons problem not mine, BUT I want to know how or what I could do to see my grandaughters. She will poison them against me I know that as she is a very vindictive, vicious woman I have seen and heard her in action.

Sounds like a really sad and rough situation. Your only recourse is to have a heart to heart with your son and possibly daughter-in-law to discuss how you feel.
If you do, try not to come off as accusatory, it will only serve to cause them to be defensive and might make things worse.
If it is easier, plead your case with your son first. Then all you can really do is hope they will understand, show some compassion, and start to make more of an effort.
If things don't go how you would like them to, certainly continue to be in your grandchildrens' lives as much as you are able to... cards, gifts, phone calls, occasional visits, etc.. It will pay off as they get older.

I would avoid criticising her to your son or getting into arguments over it. I'd just make him and the children feel welcome and continue to invite his wife to family gatherings, etc. Ignore her rudeness. The objective is that there will be no question in your son's eyes who is rational and reasoanble and who is imagining problems. It sounds like he will have to solve this one when he gets tired of the situation, and if you've been reasonable and rational all along, he will be less confused over how to manage a difficult situation.

Just wondering where you live and if you have a grandparents law or something like it. If talking won't do take them to court. I know it is harsh but it is leverage. My husbands oldest son is married to a mentally ill little girl. I mean she is off her rocker. Anyway, she doesn't like us for whatever reason so we hardly ever say the grand kids. My husband wouldn't do anything to step on anyone's toes and we talked till we were blue in the face. Finally, I called a lawyer and we sued for grandparents rights. It is an option at least.

You characterize her as mean, vindictive, a control freak, argumentative (starts all arguments too), she is delighted when she blocks you from seeing your grandchildren, she is impossible to get along with, she rules (her and your son's) roost, and she is going to poison the grandchildren against you. Apparently you characterize her mother as also being mean and vindictive.

Wow.

What have you tried to do to assess the real problem, and that is, the lack of any cordial relations between you, and your daughter in law. This wedge between you and her, is causing more wedges (between you and your son), and eventually will bring the grandchildren into this mix. Do you have your son's father in the picture- your husband? What does he say about this, or is he between a rock and a hard place like your son is.

What was she like when you first met her. What was her general demeanor in your home, during holidays, phone conversations, sharing of information. Was there ever a friendship, or did you decide (as per your description of her now), that instantly you disliked her.

If you can, put yourself on the outside for a moment, and see if you can spot or pinpoint any fault, or assumption, or unjustified (even to a small degree) impression of yourself, from your own behaviour toward her.

It is easy to see how you judge her, how do you think she judges you. Have you done things that you have regretted along the way with her? Was there one event, or a series of events that led you to the conclusions you have about her (and her mother). Surely you didn't start this way when she was new, before they were married.

Can you paint a little more in this picture to balance your opinion of her, vs. how you came to this point?

Jake I think you raised some really good points. I hadn't considered this but gayem, perhaps you should set aside your bad feelings and really consider how you have treated your daughter in law. Have you made every effort to understand that she's been raised in a different home and not the way you raised your children? Have you gone out of your way to welcome her and make her feel comfortable? Have you respected her relationship with your son, and encouraged the welcomed their marriage? Have you respected her privacy and right to make her own decisions on parenting since she's become a mother? If you only see her faults, do not like anything about her, can find absolutely nothing positive about her, I can see why she'd want to avoid you and have you away from her children.

If someone so strongly disapproved and disliked me, I would not want to be around them, either, nor have them around my son where they might trash me and try to turn him against me.