You think the grandparents are going to say to the child, look your mothers are abnormal, hell they don't even have a clue about how their own daughter turned lesbian yet parental guilt may play a role in their acquiescing to this 'unnatural' to say the least, turn of events.
No, Sam’s birth circumstances are less than optimal, but so are many others. Check out illegitimacy rates among statistical cohorts.
However, think of your own upbringing: you likely had aunts, uncles, grandparents, cousins, and FAMILY farther out than just your nuclear family/household. IMHO, the ultimate adolescent relationship is that with the grandparents: both are in physiological transition, and, while the grandparents are getting over “empty nesting”, they are rejuvenated in support of grandchildren and have much wisdom and sympathy to pass along.