How to preserve old letters

Yesterday, I got a package in the mail from my Sister. When my Mom passed away three years ago, we found a box full of letters she had written to my Dad before they were married. These letters were written to him, when she was just 15 years old. They met in Michigan at an apple orchard, as both her family and his family were migrant workers. They followed the crops and made money that way. Well, evidently, her family went west to Washington, and Dads family went south to Florida, and they kept in contact through letters.
Well, I started reading those letters yesterday, and sat here and cried. It was so touching to see how much my Mom missed my Dad, and counted the days till she was finally 18, so she could do what she wanted, and be with him. Well, she finally turned 18, and they were both once again in Michigan, and she told her Dad that they had to get married. She did not say she was pregnant, just that they had to get married, so her Dad gave her a same day shot gun wedding, and 9 month, 9 days later my oldest Sister was born. She was not pregnant, but they had a honeymoon baby for sure. She just told her dad they had to get
married, because she had waited long enough to be with my Dad.

Well, these letters are something I want to keep forever, so I am asking how do I preserve them, so they won't fall apart. They are still in the 3 cent airmail envelopes too! So if any of you know how to preserve them, please point me in the right direction.
I have never done any scrapbooking or anything like this, so it this is all greek to me!
Will they be preserved if I place them in like sheet protector sleeves?

Any and all help needed.
Thanks
Linda Kay