Hi A.,
I had this same worry with my daughter, I kept comparing her to my niece who was using full sentances at like 18 months. Everyone kept telling me that kids are different, they talk differently, she is fine now, she still is hard to understand sometimes, but she is getting better, and I figure as long as she talks for kindergarden (she is 4) next year she is ok.
My mother-in-law was telling me this story about her daughter, and son (my husband) they are 22 months apart, and my husband never talked, they couldn't figure it out, untill one weekend when he was two they left him with his grandparents, and not her. And he was chatting up a storm. They figured out, that his sister was doing all the talking for him, so he never had too. So maybe with all the other siblings around, he hasn't had to use his voice so much and is no getting used to it. Play sound games with him. and talk to him a lot, and ask him questions. I also made my daughter repeat herself when I couldn't understand her well. I still do that, I tell her straight up that I can't understand her, and she works at it until I can. I probably didn't help, but I hope I did, I know what it is like to worry about those things, so good luck, A.