Be nice to them from the beginning. There's many women out there who were dismissive of their son's girlfriends, and often downright hostile to them without even thinking of the long term impact that will have once that girl you cruelly dismissed and were rude to in the early years becomes the wife and mother to your grandkids.
My DM was very hostile to my brother's girlfriend at the time. Her criticism got really awful right through their marriage and early baby years, and then DM was sitting at home crying that she couldn't visit her GC more often as she didn't feel welcome in their home. DB used to bring the kids regularly to visit granny but her earlier behaviour meant that they set boundaries to protect SIL from her venom and all that did was backfire on DM.
A cousin used to sort our her son's house while he worked away. When he moved in his girlfriend there was a massive standoff when the GF didn't really feel comfortable with her boyfriends mother coming in unannounced and moving stuff around or redecorating. My cousin stood her ground, and it ended very badly for her because why would you trust a woman who's told you to your face you are nothing and that she'll get rid of you from her son's life with your kids?
My MIL was lovely. She was welcoming from the very beginning. I was loved by her son and once she saw he was happy, I was treated like one of the family. When I was expecting, she was delighted. When I had the baby she texted and asked when it suited us to meet the baby.
She praised me as a mother, and the things that were new since her day, she was interested and supportive. She respected our family unit and in turn, was welcomed in as a valued and beloved granny. I felt she was proud of me and happy that I was in her son's life. In fact, I was closer to her than I was to my own DM and she was by far the more attentive granny as well.
Even when she didn't get it - for example another SIL was a vegan and it baffled this Irish country mammy, she rallied and provided vegan dishes without comment even though I know she thought it was all "daft shite" 
Jesus, I'm welling up now, I miss her so much.