First off, bless you for taking this little girl in. What a big change for all involved. Just try to remember that she's had a huge upheaval and not use to 'sharing' with a sibling. I would start off not ever letting them be together unless closely supervised. You've got to be the boss and let her know as soon as she does something not allowed. Do not allow her to bully Mignon, step in, seperate them and correct her. For my kids, a firm 'bite' with my fingers on the back of their neck and a "sshhhh!" was enough for them to understand. Now I just snap my fingers and shhh and they'll stop. (And of course, give me the "But he started it!" look
) Feeding time should be closely supervised until she gets the idea that she's not the boss.
At 10 months, she can be re-taught things. It's just going to be up to you to enforce the rules and show her a lot of love and praise when she does things right.
Good luck!