How do you know?
Considering all the other things they do to nationalists, why is this beyond them?
The Lemkos family is not that important. I haven't heard of them before for example. The stakes of organizing a hit for such reasons are insanely high. Then outsourcing it to Arabs- just as insane. It means the operation becomes too complex and nearly impossible to contain. If the target is some arch terrorist or a nuclear scientist that might worth the risk and trouble, but a young Jewish lady from Tekoa- no sense at all.
I don't care how evil shabak is, they are not entirely morons. Another thing is, Jews don't kill uninvolved family members just to send message. It may be standard practice for drug cartels and Islamists, but not Jews.
Furthermore, who gave the arab terrorist organizations information on the Lemkos family?
I don't think it was targeted at the Lemkos family, the murderer wanted to kill Jews.