Was it confirmed that Mom Lalonde was adopted by Grandpa? Because just going by the photograph, to me it feels like she was probably their babysitter. I mean, Joey and Jude are 90s kids, and Mom would have been a teenager in the 90s
Her being adopted by Grandpa Harley is entirely speculation. I choose to believe it based on the evidence given, but you are free to believe whatever you like. Also Mom Lalonde would be in her twenties in the 90s.