OP, I had this with my DM.
Mine was the other way round though, 2 boys, 2 yrs apart, DM had a very strong bond with DS1 (1st grandchild in family), she felt that me taking care of the new baby (who turned into a very demanding toddler) was me neglecting DS1.
Just be secure in yourself, you know your children, I think when they are this young then it ebbs and flows as to who commands the attention most at that time. A baby and a toddler are demanding, I honestly don't think that you will be doing anything "wrong" or favouring one over the other. It is sad that you are now double checking yourself all the time.
FWIW I see a lot less of my Mother now and life is better. I think your DM needs to let you find your own feet, she is hardly being helpful, is she?