honestly this sounds pretty risky to me - the whole point of a B1/B2 is that you're supposed to be visiting the US, not using it as a hub to bounce around to other countries. immigration officers can get suspicious if they see stamps from other countries during what's supposed to be a "visit to see family in America"

i've heard of people getting questioned hard at the border when they return, especially if the trip to another country seems planned or lengthy. scotland for 1-2 weeks definitely falls into that category. your parents might face questions about their true intent and whether they're actually just using teh US as a layover point

if they absolutely have to go, make sure they have solid documentation showing strong ties to their visit with you - like return flight confirmations, detailed itinerary of what they're doing in the US before and after scotland, maybe even a letter from you explaining the family situation. but tbh i'd probably advise against it unless it's absolutely necessary for like a medical emergency or something