I am POA for my dad, and our youngest sister feels I should always include her in decisions concerning spending money on dad's house. How do I handle this?

Our 93 year old dad is in a NH due to a stroke. We have a special needs sister that is a middle child. The youngest sister is living with the special needs sister. I am POA, the oldest child and also legal guardian of special needs sister. Recently we have had to put a lot of money in to dad's house. We have used VA pension payments to make these repairs and also pay for dad future funeral arrangements.  The youngest sister feels that me and the next oldest sister ( also legal guardian) are making decisions about dad's money that she wants to be included in. I have no problem with that except she does not follow through on anything. We recently had new flooring put in dad's kitchen, hallway and dining room. She was asked to please move things our of those rooms except the heavy hutch. She did not move anything and said she did not know what the big deal was. I should have reminded her. She always wants what is more expensive instead of what we can afford and doesn't like our taste in anything. I have have heated discussions with her about things and am pretty much done. POA is not a nice job but I am doing the best I can do and always include my sister that is second in line. How do I handle this. Any suggestions?