According to Ancestry its online trees do not have that functionality.
https://support.ancestry.com/s/article/Merging-Family-TreesThe best you would be able to do merge duplicate people in your own tree or copy people (individually) from other people's public trees into your tree like when dealing with shaky leaves/hints.
https://support.ancestry.com/s/article/Copying-People-from-T...Here is the main support help page for Ancestry Member Trees
https://support.ancestry.com/s/topic/0TO150000004c4wGAA/memb...Even though online trees can be shared with others via an invite as guest, contributor or editor level, only the owner of the online tree can download the tree. Online trees can only be downloaded as gedcom files. Gedcom files can only be uploaded as new trees not into an existing online tree.
https://support.ancestry.com/s/article/Sharing-a-Family-Treehttps://support.ancestry.com/s/article/Uploading-and-Downloa...So to get her info into your tree:
- assuming her tree is public or you have access to it, you have to individually copy/merge each person from her tree into your tree
- assuming she is willing, you have to get her to download her tree into a gedcom file so that you can upload it as a new tree under your Ancestry account and then you would have to copy/merge each person from your prior existing tree into the new tree made from your sister's gedcom file
If you both want to just work on one online tree, then decide whose tree it will be so one person owns the tree and the other is an editor.
There are just two genealogy desktop programs that sync with Ancestry Member Online Trees, Family Tree Maker 2017 and RootsMagic 7. Each program syncs/interacts a little differently with the Ancestry online trees. Syncing (each program has a different name for the process) allows you to keep an online tree and a desktop tree in sync so a change made in one is transferred to the other either automatically or manually (meaning you decided when to perform a sync) without having to create/use a gedcom file. Generally desktop genealogy tree programs have more features than online trees so with either program only certain aspects of the desktop tree and online tree sync/keep updated.
https://support.ancestry.com/s/article/Family-Tree-Maker-FAQ...Because the two member online tree are owned by two different users it would still involve your sister downloading a gedcom if she does not also have her online tree synced with either FTM or RM or if she does then her making a copy of her FTM or RM tree file and sharing that with you to start your own tree with which ever program she uses if she used a desktop genealogy program. (Then you can sync your tree to a tree file in the gene program and then merge to the two files in the gene program.)
There's no instant route. Just figure the path that is easiest for you.
Family Tree Maker 2017 Support area
https://www.mackiev.com/ftm/index.htmlRootsMagic 7
http://rootsmagic.com/RootsMagic/Features.aspx