You don't need to justify why you want to copy profiles from one tree to another.
You're not only using the FREE Ancestry.com online tree program, you also subscribe to use the research databases correct? You attached links to records and have media (photos) attached to profiles, correct? I would venture that moving to another site (the suggested geni.com), is not an option you would want to entertain.
Two or more persons can work on the same tree on Ancestry.com. There can only be one "tree" owner, the others are invited and given editor status. Researching databases is limited to those who have paid subscriptions.
Since it's a relatively small amount of profiles, I would use the on-line program, rather than downloading, merging, uploading to desk-top program, and back again to Ancestry. Whatever you do, it's going to take some time. Copying profiles tree to tree might seem laborious, and frankly is, but you'll hone a skill about this tree function in the process.
Work on one tree at a time, copying from A (S-I-L) Tree to B (YOU) Tree. When B Tree has all the data who want from A tree, then A tree (S-I-L) can copy from B Tree. If you have editor status (it sounds like you do) on your S-I-L tree, you can do this for her.
To start profile copy process, you'll need *one same/common person* profile in both your S-I-L and your tree.
1) In your S-I-L tree, go to that profile.
2) On the profile page, pull the menu "More Options" down, and find "Save person to another tree" link and hit that.
3) In the pop up box, see the link "change tree" and the pull down menu, find YOUR tree in the list and choose. Now type in the name of the person, it pops in the field and hit attach.
4) On the "Review" page, the profile for the person in your S-I-L tree is on the left, the same profile in your tree is on the right. Any other profiles, those for parents, spouse, and children associated with the profile in your S-I-L tree are also listed in the left column. You can tick off all those you want to copy to your tree. Be aware the system may not alert or show you if any of these profiles already exist in your tree. Keep another tab open to your tree in the *list view* to check for all profiles you're adding, before you copy. This prevents duplicate profiles.
If there are attached links to records or media (census, photo etc) to profiles in your S-I-L tree, you can easily attach them to the newly copied profiles in your tree.
1)In your tree, on a profile page, hit the Member Connect bar.
2) The profile in the tree you see will be your S-I-L. Any links records, including media (photos, stories) attached to that profile will be listed. Hit any record link, you'll be taken to the record extract, photo, etc. On the left side of the the extract page, hit the link, "save record to (name) in my tree".