no your parents can sponsor your wife and your stepchildren, i've done it myself, my mom sponsored my wife and stepdaughter.
yes explain that your sending £300-400 a month, and once they are in the UK you'll no longer need to send the money, and you'll be financially better off as you will not need to pay 2 lots of bills.
what i would do, would keep sending them the money, but borrow some money off your parents each week/month, and live off that leaving as much money in your bank account, but don't over do it, or they might wonder what your living off !
