Once the co-op is purchased, it should not be sold out from under her as long as the maintenance, mortgage and all other payments are made in a timely manner. There are no "property rights" in a co-op as there are only shares of a corporation that are being purchased. As to who pays these fees would be between you and your sibling.
If you have any further questions, please feel free to contact me.
Good luck to both of you.
Arlynn B. Palmer, ESQ, CBR,SRES, CMS,CNS
Daniel Gale Sotheby's International Realty
Cell #: 516-410-3594