Fiction book

This book is about a girl who goes to her aunt and uncle's house. They live on or near a snow covered mountain. The girl's cousin, who is a boy, isn't nice to her. I also think the aunt and uncle had some farm animals, and the boy had a dog. At some point in the story there is a guy with a gun who goes after the children. There is also an avalanche on the mountain, which buries both children. The girl is able to get out and then uses her cousin's dog to help rescue him. He is then a lot nicer to her and he said the reason he was mean to her was because she looked like another girl who was mean to him, I think. I also remember character names, I think the girl is named Molly and the boy is Glenden.

Nightmare Mountain by Peg Kehret.
Run for your life! It should have been a "dream" trip. Instead, Molly Neuman's vacation on her Aunt and Uncle's llama ranch in the wilderness of Washington becomes a nightmare. Cousin Glendon hates her on sight. Then Aunt Karen falls ill and is taken to a distant hospital. Molly and Glendon are alone when she discovers a llama thief in the barn! Armed and desperate, he drags them to the top of the mountain and triggers an avalanche with a gunshot when they try to run! The thief escapes and they're left to battle a tidal wave of snow. Then Glendon disappears. Now it's up to Molly to find him and save them both from being buried alive!

Thank you :)