Force Messages to send as SMS on Yosemite

I have an iPhone with iOS 8 and a Mac with Yosemite, which means I can send SMS messages from my Mac (which I absolutely love). However there are some special cases that it doesn't handle as I would like it to.

For instance, my sister has an iPad with iMessage set up, but she uses an Android phone (i.e. no iMessage). If I try to send her a message from my Mac, it usually (not sure if always) sends to her iMessage account. This is a problem since she doesn't always have her iPad with her so sometimes she doesn't see my message for a long time. On my phone my messages with her are split into two threads, one for iMessages and one for SMS messages, so I always reply to the correct address. However, on the Mac they are merged into one thread. So sometimes she sends me a SMS from her phone, which I receive on the Mac, but when I reply it sends an iMessage.

Is there a way to either force Messages on the Mac to send to a specific address or to split the thread as it is on the iPhone?

you can choose where your message will go: click on the little down arrow right to the recipient's name, and select the destination Message or iMessage (or even other chat system like jabber or bonjour).