Is it normal for them to forget a son-in-law but not their grandchildren?

Memory will even change from day to day. Some days she may remember her son and other days she won't. The grandchildren are "novel" in that she may not see as many children as adults, but she will - sadly - eventually forget who they are, as well. People generally remember faces longer than names. They forget the relationship that people have with them but may still remember the face and that the person is "there" for them. So, as Jeannegibbs said, there is no "normal" and being forgotten shouldn't be taken personally.

Please keep coming back to ask questions and for support. There is enormous wisdom within this community.
Carol

If grandma has dementia, any kind of memory pattern is "normal." My mother doesn't recognize one daughter-in-law, even though they were once pretty close and Mom spent a lot of time at their home. Who knows why? Maybe she just looks a lot different now. I don't think that the forgotten person should take it personally.