Angels Can Take Any Form They Want

Angels are spirit beings, so when they need to talk to us, they can choose the shape which is right for the job.

Unfortunately this can also be used for evil deeds. For example, it is satan himself who will impersonate Jesus.

Satan will take the field and personate Christ. He will misrepresent, misapply, and pervert everything he possibly can.
- Ellen White, Testimonies to Ministers and Gospel Workers, p. 411

And satan's angels will also impersonate lost family and friends.

It is not difficult for the evil angels to represent both saints and sinners who have died, and make these representations visible to human eyes. These manifestations will be more frequent, and developments of a more startling character will appear as we near the close of time.
- Ellen White, Evangelism, p. 604

In the last days the deception will be off the charts.

Let's stay awake.

By the way, the verse below does not mean that angels pretend to be strangers. They just happen to witness it.

Be not forgetful to entertain strangers: for thereby some have entertained angels unawares.
- Hebrews 13:2 (KJV)