Why is it that when you say "no" to a person, they act like an ass? (details inside)

Do you ever find that when you have to turn someone down, or say no to them for whatever reason, they act like a jackass?

Here’s my situation:

Saturday morning, I was primped and ready to head off to my grade school reunion. Then, the headache started, followed by the chills, aches, swollen glands and vomiting. So, the reunion was a no-go. Bummer. I climbed into bed after upchucking a good part of my innards and all that I wanted to do was sleep. I was weak as hell. Then, the phone calls came. I let them go to voice mail. Four calls were from my one needy bro (all that he wanted to do was chit-chat. Nothing urgent). I slept a good part of the day and when I got up, I decided to call him back. I got his voicemail. I then said to him “S.., I’m quite sick today and just need my rest. It difficult for me to get up. I noticed that you called, I will call you tomorrow”.

He then calls again an hour later. I get up to answer the phone. I asked him if he got my voicemail, he said yes. He asked me what was wrong. I said that I’m quite sick and that could we chat tomorrow. He said “Okay” and hung up. He was pissed. I called him back asking if he hung up on me and he hung up again. I know the dude has not much going on in his life (he’s on disability) but, he didn’t need to treat me that way.

My sister called this morning. It’s our long weekend here (Queen Victoria day). Her husband is away and she wanted to know if she and my niece could come over. I explained that I was sick. She said that she understood, but, that Maddie was having a terrible weekend with no friends to play with and that she could just come and play with the pets. My sister also said that she needed to get out of the house for a bit and that there was nothing to do where they lived.

I reluctantly told her that if she wanted to come here (for Maddie) she could. I mentioned that I wasn’t in the mood for visitors, but, it’s beautiful out and if she needed to get away, come here. I’m really hoping that she doesn’t. I wish that I could have said no, but, I didn’t want to have to deal with the sulking.