If what they owe, plus delinquent payments, etc., is more than the house is worth, while you could still buy it, it wouldn't be a good financial decision. (But because they're family, you still might want to do it.)
If what they owe, plus delinquent payments, etc., is more than the house is worth and you didn't want to pay that amount, you could try for a short sale. That would involve offering less. Such a transaction would need the lender's approval, since the lender would be losing money on the transaction.
If the bank owns the property, if there's a redemption period, you could redeem the property.
If the bank owns the property, and there's no redemption period, you could always make an offer to the bank. However, it's unlikely the bank would accept. At some point, they will list the property with a Realtor and it'll go on the MLS. (The bank wants to minimize its loss, so it would prefer to offer the house on the open market, rather than sell it to someone in a private transaction.)
Hope that helps.