You have to remind your husband here of the Mother's day thing. First you celebrated 3 mother's day, but you were busy the first 2, either you were pregnant, dealing with a baby as a first time M. and the 3rd mother's day, you have a toddler and pregnant again, so be patient with him. It might surprise you, but he may actually have never thought of giving his dear wife a mother's day card in the heat of everything, so remind him you would like to be considered as well.
As for your inlaws not doing it, that I don't know their reasoning. Since your birthday and Mother's day fall around the same time, it's possible they don't have it on their radar or you have not made a big deal of it.
No, do not stop sending Father's day cards either. It will get better over time when the light bulb goes on in his head and the kids get older. Yes, you deserve to be recognized as a M....
PS: You've only been married 6 yrs, so I am sure before the kids, it was all about your marriage together, so he is still young at this routine...remind him.