I have a similar issue but it does not seem near as extreme as what your sister is experiencing. And I have had no good suggestions on treatments until recently when I had toe surgery to remove a part of a nail.

Long story short, the podiatrist recommended I use rubbing alcohol on the inside of my toes.

The other issue is that it is a simple untreated fungus. I received medicated lotion. I can use it up to three weeks, twice a day and the chapped roughness and yellow toenails turn back healthy. Then I take a break. I have found that my fungus cream has worked so well that I no longer have to use it nearly as much. And the rubbing alcohol has been a great solution to my between-toe blister.

I don't think it is MS related, it is easy to blame everything on freaky MS and all its weird manifestations.