Okay, so, I think that Keith, Lance, Pidge, Hunk, Matt, Shiro and Sam have been in space for a little over a year. Two if you count Matt and Sam getting captured by the Galra with Shiro for about a year or something. Anyway, time moves a lot slower in space than on Earth. Especially if you're far away. Because they are thousands of light years away from Earth which means that it's probably been around 100 years or so since they left Earth on Blue. Even thought they've only been away for 1 1/2 or something.

I was watching this anime earlier called Godzilla (it's awesome, you should check it out), anyways, they were traveling in space for about 22 years and were 11.9 light years away from Earth. They thought that 1,000 years had passed since they left, even though they've only been traveling for 22 yrs. It's because time go slower in space. Then they find out that 20,000 yrs had actually passed.

So, to confirm, since they're probably a few thousand light years away from Earth, it's safe to assume that around 100 yrs has passed on Earth, but for them it's only been just over a year.
That means that all of their families are dead. And even if they did return to Earth eventually, everyone and everything they knew and loved would be long gone.

I became very sad when writing the last part. I just can't stop thinking about if my theory is true. And how sad they would be if they returned to Earth and saw that everyone they knew were gone forever and that Earth had changed drastically.