Hi there
Gemindiva77...welcome

There are so many of us here who are just having to suspect BPD as being the problem of our troubled child...because of one reason or another, no formal diagnosis.
I imagine it was quite a road you travelled until you were given the results of tests done with/on your daughter. The more you read in posts of others in this forum, the more you will find that there is a wide, wide range of BPD behaviours. It is so important for one to realize that there are no quick fixes...no magic button to push for situations to get to the "normal" one would want. It is even more important to realize that your shared future might never be the one you want...just different...but can still be okay.
While it seems that medication does indeed help with some (if they are compliant and will take the medication), that has got to be a hit-and-miss endeavour for doctors/physiatrists to manage. It is a which-one-to-administer...then...dosage to work on. Hopefully all that will kick in sooner than later in your daughter's case.
For sure that is a big problem having a younger child watching and learning from a troubled, older sibling. I can see why you are in a rush to make sure he is kept on the straight-and-narrow as he absorbs and reacts to his environment. To say the absolute least, it is unfortunate that we are in the times we are with services cut back or totally cut off.
Not to say that you aren't already doing your best...but you are going to have to work all the harder to work on skills in how best to deal with your daughter. Take full advantage of all that can be found on this website...with links to more. Oh yes...all this is time-consuming but good results can follow. Your son needs to see you setting boundaries with her and then sticking to them. Then again, while some boundaries are written in stone...others can be negotiated... but in a calm manner. If he is privy to that kind of interaction...might be yet a good lesson for him to learn.
You are a "Newbie" here and I want to point out that an answer to a post should not be considered a dismissal. This is an interactive forum. While each of our stories differ, each post hits a nerve with someone who finds themselves in a similar situation. What you have written has hit a nerve with someone...letting that person know that they are not alone.
Hope you keep sharing, Gemindiva77. Let us know what has/hasn't worked with your daughter. How are things going with that 5-yr old? With all that said...you share as much or as little as you like...no problem! Hope, too, that you get comfort in reaching out to support others here.
Wishing you and yours continued immunity with happier days just over the horizon.
Huat