I think it's risky. It's not the same situation as being refused at the border, but I've seen people rejected for visitor and other nonimmigration visas (B1/B2/J1) based on how much time they've spent previously visiting the US.

Obviously we can't give legal advice, but it would help to know more about your in-laws. What's their nationality? What visa (or visa waiver) are they using for the 6 month US stay? How long are they staying in Canada? Are they planning for the 2-3 day connection to be part of whatever visa status their 6-month stay is?

Personally I wouldn't have them do a 2-3 day connection from Canada-India. Go for the quickest route possible or else it might look suspicious to CBP.

http://travel.state.gov/visa/temp/types/types_4383.html information on the C1 transit visa, if it's relevant.