Learn Arabic (Modern Standard): الأخ مشغول الآن.

In Arabic, sentences that describe someone or something don't use a word for 'is' or 'are' in the present tense. You simply put the subject (الأخ, the brother) right next to the description (مشغول, busy), and the 'is' is naturally understood.

Why does الأخ have 'al-' attached to it, but مشغول doesn't?

This is the secret to making an 'is' sentence in Arabic! The subject usually has ال (al-, meaning 'the') so we know exactly who we are talking about. The description word stays indefinite (no ال). If you put ال on both, like الأخ المشغول, it would just mean the phrase 'the busy brother' instead of a full complete sentence.

What is the wavy line over the alif in الآن (now)?

That wavy line is called a madda. It tells you to stretch out the 'ah' sound. Instead of a short, sharp 'a', you say 'al-aaan'. It is actually a combination of two alifs that got merged together to save space!

How would I use the question word من (who) to ask 'Who is busy now?' based on this sentence?

To ask 'Who is busy now?', you just replace the subject (الأخ) with the question word من (man). The sentence becomes: من مشغول الآن؟ (man mashghool al-'aan?).

How would this sentence change if we were talking about a sister instead of a brother?