Are Skyler and Marie really sisters?

Ok so this may sound like a dumb question but to be fair they don't look alike at all. Then it got me thinking that maybe Skyler could be a half sister or adopted sister.

During Marie and Skyler's no-talking phase Skyler tells Hank that Marie was always the one getting all the attention and everything. This could be why Marie is always wanting attention because she feels she didn't get enough once Skyler came along.

This is a huge stretch I know but honestly I wonder. They couldn't have picked two more dissimilar looking women to be sisters. Just saying. Thoughts?