sprinkle it's really tricky. I was guilty of having too much when dds were young and it was heartbreaking thinking of the money wasted when they outgrew all the beautiful clothes before even wearing some of them.
I've stopped keeping clothes from dd1 for dd2, they both like different styles and tbh it's cheap enough to buy dd2 things she likes from vinted/car boots or sales etc. And if your mum does like buying your girls so much it may be a good idea for you too. I just kept expensive things like branded school uniform (if in decent nick)
As a start, go through the wardrobes and find the 8-10 best of each category - t-shirt, leggings/jeans/trousers, shorts/skirt, dress, long sleeve top, jumper/hoodie/cardigan. A couple of occasion/party outfits. All of these can be mixed and matched for all year around layering up and allows at least 2 weeks worth of outfits without washing and lots of choice. If your girls are anything like mine they will wear the same 2/3 outfits over and over again anyway!!
1 or 2 Thin rain jacket and 1 thick waterproof coat (up to you whether you keep one or 2 of these, we usually had 2 waterproof coats when they were young - one school and one outside school). We also had another jacket (furry in winter and bomber jacket for summer). 2 × winter hat, gloves, scarf, 2 summer hat and sunglasses. 1 or 2 swimsuits - we had a swimming costume for pools and a UV sunsuit for the beach.
School uniform have 5/6 sets including a few extra pairs of tights. We used to pack away the summer dresses each year too (only ever 4/5 of these as they wash and dry so quick). Underwear and socks we usually had around 15-20 of each due to accidents/frequent holes!!
This is just a start and you might find actually you don't even need or want this much. (We used to have over double, almost treble this amount at one point it was ridiculous! And actually more stressful for the kids getting ready in the morning trying to find things or putting clothes away)
Once sorted sell the extras on vinted - really nice/branded items separately and the rest in aged bundles. I usually do boden/zara and party dresses separate. I do £3-5 an item and a bundle of casual clothes for about £7. Usually sells very quickly. Especially if you set up bundle discounts on your account. If you are desperate for space maybe give the items a set amount of time then charity shop/olio/Facebook free site them.
Then you could you use the money towards a day out or experience for the girls? Or split and put into their accounts? We recently raised enough to buy each dd an ipad for their school work. (Plus more in their savings)
Then from now on - one in one out. If you buy a new outfit, get rid of one that's outgrown or getting tatty. We now have a bag going for the rag man (stained/holey/worn out), charity shop and a vinted selling bag. Once full get rid.
I'm sorry for the essay, I hope it helps. 😊