Hi Toby. When you're completing a merge, you'll be brought to a page that lets you choose which to choose for things like name fields, birth and death dates and places, etc. This includes parents, because a profile can only have one set of parents attached. (This is not an issue for spouses and children, because there's no limit to multiples of those.)
If only one profile has parents attached, the default will be to keep them. If both have parents attached, you'll have to choose between them, or, if they're the same, merge or match them.
Siblings are defined as profiles that have the same parents, so siblings will remain attached as long as the parents are.
I hope that clears things up!