I just need some opinions

This is a bit long so bear with me please.

My husband, I and two kids live with my father in law. We own the home and he lives in the in law suite. In December he moved his on and off again gf In with him. She used to babysit a two year old where she used to live. Last month on a late Sunday night- 3am she shows up with the little girl. She stayed 6days. On the 6th night she kept crying that she wanted her family. The next day Layla- the girl is gone. I’m assuming they brought her back home. This past Sunday morning Layla’s back. Now I know the little girl has both parents that love her but I don’t know what’s going on that she’s here so long every month and constantly asking to go home to her family. Part of me feels like the gf is pushing her boundaries. Anyways, my kids are 3 months and 1.5yr they caught hand foot and mouth Sunday. My daughter- 1.5yr has had it before and it’s not pretty to say the least and I know it’s very contagious. Earlier today gf comes in down stairs and sends Layla up and tells her loudly from downstairs to tell me she’s staying up for a while. For starters I didn’t agree to babysit her so gf can go smoke her weed like she wanted to do. Second of all I know damn well Layla’s parents wouldn’t appreciate their daughter coming home with h, f, m. So I sent her back and told her I’m sorry she can’t stay up here. Main living area is upstairs. Gf then proceeds to ignore what I said and sends her back up. I then put my foot down and say no again very firmly. Gf then goes to complain to father in law that I won’t keep Layla for a few hours and then proceeds to tell Layla every 30 minutes that Maddie’s mom is very mean. And it’s been going on for about 5 hours now. So I guess my question is, am I being very mean and was I wrong to send the child back to the gf?