just because the money is 'for her' doesn't mean that she should get it to spend freely....a reasonable amount should be used for her care and feeding....if there are three people in your household then 1/3 of the rent is housing her....ditto the utilities and food etc...it should come out of that ssi check......and some should be put away in savings for her when she's older (college?....first apt? etc)
my three kids got a taste of this same thing when they were growing up...after their dad left us, he married a woman with two kids... each of her kids (young teens) got $1000 a month because their father had died....i got NO child support from my ex AT ALL and barely made ends meet every month (and frequently the ends didn't meet, sometimes i'd even be working two jobs)... he was working two, sometimes three, jobs to support her and her kids ....and each of her kids were given their ENTIRE social security check each month to spend as they pleased...
i suggest sitting down with your daughter and showing her how much it costs to run your household and gently pointing out how it's not fair for her to have so much spending money when a good chunk of that money should go to her living expenses, since that's what it's really intended to be (just like child support)....ask her to help in setting up a family budget and have her help decide what is reasonable to her to save and how much of her monthly check should go into the family budget...hopefully, it is done is a gentle and non-confrontational manner, you can work something out...
you're really not doing her any favors allowing her to grow up thinking she can live high on the hog while the rest of the family suffers....and as your son grows older he will come to resent her ...no matter how hard you try to prevent it....just ask my kids about their step siblings...
D.
ps....from the social security website:
About 3.8 million children receive approximately $1.6 billion each month because one or both of their parents are disabled, retired or deceased. Those dollars help to provide the necessities of life for family members and help to make it possible for those children to complete high school. When a parent becomes disabled or dies, Social Security benefits help to stabilize the family’s financial future.