How do I tell my friend I'm interested in her brother?

Hello! I used to live in an apartment building with my ex. There was a couple across the hall that we used to hang out with. They were really cool and I met the girls family and saw them a couple times while I lived there. The girls name is Emily. She has 3 brothers. One around my age named Mark. Mark came over to visit his sister one day when my ex and I were over. He talked to my ex like he knew him one day. My ex and Mark had a mutual friend. I think they played xbox together is all. So my ex and I had moved and recently broke up. I went back to the old apartment bulding to see everyone one day because I was feeling down. Anyway Emily and her boyfriend were outside so we got talking. She told me I should get to know her brother Mark more because she wants to keep me around and he needs a good girlfriend. I wasn't sure what to say out loud so maybe I didn't seem excited enough. She hasn't brought it up since but she brought it up a few times that day and said she needed to plan dinner with him and I. I'm a little worried that if he knows my ex he might hear negative things about me. Also I tried to add Mark on instagram months ago when I was with my ex because I knew him a little and he didn't follow me back.. is that a bad sign? I think I'm looking too deeply into it. Anyways I want to ask my friend if she was serious or not about her brother and I. He actually seems like a great match! :) So what should I say or how should I ask? I don't want to seem desperate, and he doesn't know about this I'm sure so I don't want him to feel awkward. Thank you!
How do I tell my friend I'm interested in her brother?
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