First- have they had a regular doctor's checkup lately? A word in the doc's ear ahead of time about the problem ( preferably a female doctor so they don't get too embarrassed) could give you some backup in dealing with this issue.
It sounds like their mom must not care, or she would be on top of this! I agree with other posters- you can approach it from a health standpoint, a popularity standpoint, a self-pride standpoint, ... but I think you have to speak up and stay on it, if their mom is not going to help them!
I know you want to be a cheerleader, and bless- but sometimes the 'mom' (or stepmom!) has to come down on the side of 'ok, this is just not acceptable- in our family we are proud of ourselves and take care of ourselves!"
Maybe you can make it fun with regular 'spa' nights- not just mani-pedi stuff, but hair-washing, ear-cleaning, etc. Or do a skin care/facial evening? I sell Mary Kay and if you have a Mary Kay lady near you, you could have her come over and do a special 'teen skin care' class.
Anything to make it seem fun and not like a chore- but seriously, you are a good stepmom for trying to give these girls some guidance!