A teenager girl, forced on a family trip to visit her eccentric Grandmother, becomes an unwitting time traveler after discovering a magical chaise lounge in the attic- trapped in the past, she must seek help from an unlikely source- her 14 year-old Grandmother from 1956.

I would say the revision is better but I agree with Richiev that you should label the girl so it highlights she is the main character of the two, however they seem like they will both be sharing the story time so they are both quite main.

After discovering a magical chaise lounge, an (adjective) girl and her brother begin travelling through time. When they find themselves in 1956, the siblings seek help in their 14 year old Grandmother who must help return to their era.

This attempt is a little dry but it is a tough story to boil down. I think you are on the right lines with what you are writing but a few things need clearing up. Hope this helps

I'll add this, they aren't "stuck in 1956" but rather stuck traveling through time without being able to get "home." They move through time on a magical chaise lounge by reading history books... Ultimately, they choose, by accident it seems, (more of a plan unknown to them) to travel to 1956 in a rush to get away from King Tut...

I think the revision is just a little better, but I might say, an (Adjective) teenage girl and her brother. (The Adjective would be a unique quality that describes the girl)

How about this as a try:

"After becoming trapped in 1956, a rebellious teenage girl and her brother must seek the help of their 14 year-old grandmother before they are lost in time forever"

You would still want to add a details to this logline but I hope you see where I am going with it.