NSE Investment under Parents Name - Tax implication and limits

Hello, I am an NRI hence I cannot purchase NSE or any Post office products, my wife stays along with me hence cannot purchase under her name as well. I would like to know if I can purchase NSE (National Savings Certificate) under my mothers and in-laws (Wife Parents). I understand that there is no limit on NSE certificates purchases so they can purchase. Please let me know if there is a cap on the purchase? My Mother and in-laws are senior citizens 60+ years living in India, hence they can benefit additional interest than normal. My mother is already filing tax every year as she receives rent and also pension as an income but doe not pay tax as it does not cross the limit, however, if I make this investment under her name as primary so will she attracts any tax implication or not? Any limit on her to invest in a year or no limit as money will come from outside India and I pay all the taxes overseas and can provide all proves if requested. For my in-laws, they don't file tax as they are farmers. Can I invest under their name - NSE Certificate. again any limit per financial year?