Unfortunately, there is the law answer, and there is the family answer.
The law answer is fairly straightforward - she deeds the land to you, and you leave it your husband if you precede him in death.
The family answer is different. Many families have been torn asunder by inheritance issues. Your siblings seem to think they deserve a share, which is ...
It's definitely not their land, and it probably was your grandmother's land. Title has passed to whoever she willed it to.
The three key elements to adverse possession in Washington are open possession of the land, a good-faith claim of title to the land (correct or otherwise), and paying taxes on the land for at least seven years: