My cat is acting up and I think it's because of the new kitten?

by Beck
(Alberta, Canada)

I have a two year old adult female cat who has been a beloved only child for myself and my roommate for a while now. She was abandoned by her previous owners and left to fend for herself until we adopted her, and before that we believe she was separated from her mother too early because she suckles "her" blanket.

She's got some abandonment issues. She's been our only kitty for a while, until we took in another cat, a little male kitten who needed a lot of TLC. He had some health problems and required a lot more attention than we'd anticipated.

Shortly after his arrival, our girl has taken to misbehaving, chewing on headphones, eating his kitten food, eating people food, going places we thought we'd taught her not to, etc.

She's being very sneaky about it too, creeping into a room if one of us leaves to go to the bathroom, or doing it while we're at work and getting caught red pawed when we get home.

She gets banned from a room, including one where he still may be because of his enclosure, and I'm worried any disciplinary action we take may be going into her mind as us favoring/replacing her with him.

Is there anything we should or could be doing differently? I don't want her to think the fact she's not allowed in certain parts of the home is because I prefer him more.

Does that make sense? I just want my girl to feel loved and secure.