It's always totally your choice who has contact with your DS and who doesn't. So, no YANBU.
From my pov, swearing and drug use are big red flags about who I let near my DCs.
From your description, it appears that she is missing her own son very much and, perhaps, is just feeling lonely and wanting to be involved with a family.
Others may be able to comment in more depth, but I have known a few young women who have had their DCs taken from them. I came into contact with them through an organisation I used to volunteer with.
If your brother is 19, I assume this girl is a similar, or younger, age. It is relatively easy - and DOES happen - for young and/or less educated women in, shall I say, less than ideal living circumstances to be over awed or even threatened by social services.
This girl may well have had a rather chaotic home life and, despite her best intentions and putting in a lot of effort, social services - or the SS, a very appropriate acronym - have decided that her DS is better off with foster parents.
The girl may or may not agree with this assessment - I'm guessing not. But sometimes it's best just to go along with Social Services for the time being and get at least SOME access to your DCs rather than argue with them and get no access at all.
On the other hand, she may just be a complete b**ch that doesn't care about her DS and that is why he is with foster parents.
I'm just trying to give some possible background to her behaviour so that you're not too weirded out by it.
At the end of the day, you and your DH/DP are the only ones who should decide who does or does not have any access to your DS and you don't need to give any reasons.
But, she is probably missing her own DS an incredible amount so, please, don't be too rude to her.