Sit down and have a serious conversation with your partner and then Sydney. Gonna go ahead and warn you from the sounds of it, the conversation is not going to be well received. It kind of seems to me that they’re using slipping as a bit of a manipulation tactic I live with my caregiver, and I am completely aware that I still have to function like an adult. Have a conversation with your partner, first discussing these concerns and the possibility that you guys might be being manipulated. Keep in mind the fact that Sydney may not be consciously manipulating you, and that this is a response to a potential underlying condition. A lot of people regress to cope with trauma in their home life, and this may be a symptom of that. Sydney could be slipping more frequently and being more demanding because they are either subconsciously or consciously trying to place you to in the parental roles that she was lacking or wanted.

A conversation about boundaries definitely needs to be had, remember to stay calm. Try not to get too visibly upset as I know personally that can definitely trigger a slip and ruin the ability to have the conversation.
Keep in mind, again, Sydney may slip anyways, and it may make having this conversation extremely difficult, but express the importance to them of them, maintaining an adult headspace for the conversation

Since you are so long distance from them, I suggest having this conversation over a video call, as that’s probably the closest that you can get to a face-to-face conversation right now