Whether or not family affection is stronger than romantic love is very subjective. It depends on how your family shows affection, and how your romantic partner shows affection. In my opinion, I believe both kinds of love are equally strong. I feel family affection from my Aunt and Uncle. They took me in at the age of sixteen because I could no longer live under the same household as my mother. I know that they love and care for me as they do for their own children. My aunt and uncle would do anything for me and are a big part of why I am where I am right now. On the other hand, my boyfriend of almost two years would also do anything for me. He was a big part of my freshman year when I was going through issues socially. My first roommate freshman year had turned all my friends at the time, against me. If it were not for him, I would have stayed isolated in my dorm. He offered me a lot of support and always did everything he could to make sure I never felt lonely. This is why it is hard for me to give a straightforward answer on which I believe is stronger because my legal guardians and my boyfriend have both shown me a great amount of affection in their own ways.
However, if one were to ask me this question with my parents and a previous romantic partner in mind, I would have said, neither. My mother treated me horribly and only showed affection towards me when it benefited herself, or her reputation. My father was always on business trips and sent me money as his form of affection. My previous boyfriend only cared for me to a certain extent. This proves my original statement that whether or not family affection is stronger than romantic love, the answer is subjective. Personally, I believe both can be as equally strong, as long as the people showing you affection genuinely love you.