i am rather aghast that some people have been so harsh. i cannot see at all how you are being insensitive. it is clear that you love your family- all of them- and don't want to stop spending time with them, and are just looking for a little help in this one particular area. and it's a real problem. the smell would be bad enough, but leaving a lingering odor on the furniture is a sign of something that truly needs to be addressed.
first off, i'm so sorry you're faced with this delicate situation, and totally applaud you for not thinking of cutting these folks out of your life but only to finding a courteous kind solution.
i think it's very possible the mom doesn't know. through all my years of smoking i always thought that washing my hands and taking a breath mint completely fixed the odor. ha! and while for most of us homeschooling is extremely social, busy, active and involved, i can also understand how the challenges of homeschooling teenagers with special needs could be very isolating.
you can't put it off any longer. you've got to take the bull by the horns. i wouldn't even try to approach it elliptically or find euphemisms or turn yourself inside out to be tactful. this mom has probably spent this child's entire life watching people crabwalk around them. get the kids settled (probably at her house) and get her alone for a cup of coffee and just do it. 'annie, you know how much i love you and the kids. you are so important to me and having you all in my life is a huge blessing. i've got a situation and need your help in finding a solution. now that benjamin is such a big boy, he seems to have developed a big-boy odor that goes beyond the goat smell most teenage boys contend with. it actually smells as if he's having trouble wiping, and the odor is so strong that it remains where he's been sitting. i would never dream of making him feel self-conscious, but we do need to address it. is there something about his personal hygiene that we can help him with? how do you think he would feel about having a special 'benjamin' chair that is all his (maybe paint or decorate it somehow) that has a plastic covering?' and make plenty of reassurances as to how much love them and enjoy their company. listen to her response carefully.
if she gets offended, set your jaw and keep offering love. it would not be surprising if she has a knee-jerk storm-off, but i'm betting that after she gets over the initial shock and hurt that she will really appreciate your openness and honesty. and it's very possible that she'll take it well in the first place.
good luck, and please let us know how it goes.
khairete
S.