You need to be strong and call a family meeting and air your veiws calmly and in a business like manner.discuss the fact that your stepdaughters partner must pay a proprtionate amound of money towards housekeeping,because the way it is with you struggling to provide is not viable.also mentioning this with the whole family there,he is going to find it hard to disagree b/c he must know he is wrong.or maybe he is just being ignorant and giving the amount he been asked for by your wife so you could have a word in his ear.
Good luck and let us know how you get on and try not to let it get to you .
Barry