If the reality is as you mentioned, it is permissible for you to marry the maternal cousin whom you mentioned, because
the breastfeeding concerning which there is doubt has no legal effect. It is well-known that the milk-relation that prohibits marriage is that which is due to known breastfeeding, if it is five or more feedings, within the first two years of a baby’s life. If it is not certain, permissibility is the basic ruling.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.