"I get a skirt from my sister."
Actually it's a case of ni being used with certain verbs to mean by/from - in this instance "I receive/get a skirt from my sister". Same with 習います、お母さん に 習っています。- I'm learning from my Mum. Another example of this is having some do something for you - 姉に スカート を 作ってもらいます。Literally - I receive the making of a skirt from my sister - but a more natural sounding English translation would be - My sister makes a skirt for me or I get my sister to make me a skirt.
That's not accepted because it is a set grammatical construction - in Japanese certain verbs 'take' certain particles as a rule. This is the case here for もらいます. When used with もらいます, the particle に means "by way of" - so "I receive a skirt by way of my sister". It doesn't translate very well into English. But it makes a bit more sense and translates into English better when you learn the construction verb te form + もらいます. This construction literally means "I receive ....(whatever the verb in te form is)" and is a very polite way of saying someone did something for you eg. つくってもらいます - make for me. Similarly, there is also verb te form + あげます (literally "I will give you whatever the verb in te form is) which is a very polite way of saying that you will do something for someone つくってあげます - I will make it for you. Now if we look at that first construction I mentioned (verb te form + もらいます) 私は 姉に スカート を つくってもらいました - literally this means - I received the making of the skirt by my (older) sister. は does not follow あね because あね is not doing the action - あね is not receiving or having the skirt made for her - 私 is. So あね needs a different particle to show what she is doing in the sentence. That particle is に and it tells us that あね is the person who kindly made the skirt for 私. Maybe when these lessons are finally out of Beta more lessons etc including constructions like this will be added and explained so people aren't left confused and wondering why certain particles are used seemingly uncharacteristically.