There is no excuse for a nine-year-old to be so careless with her clothing. You have rightly stated that your toddler, at two years of age, is not as messy as your SD! Clearly, the girl assumes that her socks and other items of ruined clothing will be replaced with new things; she has no reason to think otherwise. I suggest that you stop replenishing the socks that have been (deliberately?) muddied up. 

I suggest that, in the future, you hand filthy socks to your SD and let her wash off the worst of the mud. You can then run them through the washing machine to get rid of whatever remaining stains can be removed and let her wear the grubby result. Most children, if they were provided with brand new clothing any time they trashed their old garb, would be just as cavalier. Your SD will not enjoy wearing nasty, stained socks!

And all muddy items.

my sil said she trained all her kids to hand scrub their school shoes when they got dirty as they're meant to be white as per school uniform. 
 

Her husbands kids from first marriage never were taught to clean shoes and would throw them in washing machine together with clothes.

my ss did this and it's disgusting when you have kids/baby clothes to wash

Well she's 9 now so I personally would welcome her to the world of "you're now old enough to wash your own clothes' 

Oversee her over course so she doesnt tear sh*t up but I would not and do not wash step kids laudry because I'll be damned I do any more free house labor or cater to those step kids any more than I'm already doing. Basically, it's beyond my paygrade (my soon to be ex is cheap af and so he'll have to get someone else to do his kids laundry). 

If you don't want to teach her how to wash, give step daughter some grocery bags and send that dirty mess right back to her mother. Or let your DH do it.

*also get some washing machine tablets that clean all the gunk out after their beyond dirty clothes go in there.

I like the idea of sending her really gross disgusting dirty things back home with her. And if that's not an option, then definitely I would have her start doing her own laundry. I mean seriously let's all think about how easy it is to do laundry in the 21st-century we throw it all in the washing machine we throw some detergent and we hit the start button then we move things over to the dryer and hit the start button. Yes we might have to pre-spot some stains but that's about it and thirdly there's no way in hell I'd spend my own money buying new socks that should come out of your husbands money.

I've started getting into the habit of telling my skids to take home their laundry. Only the clothes that we bought them will I wash which is only like a days worth of clothing every other week. Lately they've stopped taking their clothes home like they're supposed, they throw their clothes even the clean ones I wash on the floor. When their dad sees the mess he yells at them to clean up and they just toss everything clean or dirty in the hamper. I take the clothes and toss them back  on their floor or pack them for BM to wash.Let him deal with their mess, let there BM deal with their laundry, I have a newborn and I don't need the extra load... literally.

It's time to teach your skids how to do laundry. I'm shocked at how many skids don't know this basic life skill. Make SD rinse and pretreat her muddy clothes -  hold her accountable and she'll start being more careful about such things. You may handle the "everday" laundry, but it's not okay for SD to be creating extra work for you and she shouldn't be rewarded for it with endless replacements.