A Stranger in the House

I am 22 and my younger brother is 21. We both have relationships. I used to invite my boyfriend at home all the time so my family would know him better. He's close to them now and I'm glad. My brother has a girlfriend for two years now and we were aware of that. The matter was, he's not bringing the girl at home to let us meet her. Before, we didn't know the reason. Later on, he revealed the reason why.

My brother's girlfriend was living also in the same neighborhood. She was living with her eldest sister but originally, she came from the province. Her sister knows our family and she is actually a friend of my mother but she does not like my brother for her sister. She's mad when she was hearing that my brother and my sister are dating and meeting secretly. She's harsh to my brother and hates him much for some reason we don't know.

When my brother got a chance to work on my uncle's auto-mechanical shop, he stayed in there. At night, he's the one left there to look after the whole premise. There they secretly meet. Until unplanned, the girl got pregnant. 

Her sister was hysterically mad when she knew about it. She was harsh and she even wanted her sister to leave their house. Of course, responsibly, my brother took the girl home and now she's living with us.

I don't know the girl personally so I feel like she's a stranger in the house. I can't feel comfortable when she's around. I know, in time I can adapt this adjustment. The matter is now. How can I act towards her. I don't even want to look at her because I feel like there's someone living with us I don't really know. It becomes a paranoia. I feel like I can't show off myself. I'm not saying that I am not treating her nice or I'm acting like a villain towards her. I'm not like that. I just can't talk directly to her. As much as I can, I really want to build a good family relationship between us but I don't know how to start. All I know is, I'm so excited about the baby thing!

Any advice? Thanks hubbers!