He IS speaking some...so don't be concerned. He is not incapable of speaking, just unwilling.
Late walkers don't mean they walk any less when they grow up.
Don't be concerned about the lack of words now.
Older siblings have a way of making the next born speak less...
Think about this...You are not giving the undevided attention the first born had. The first born is helping with communication for him in ways you don't realize. He doesn't need to communicate verbally as much as an only child does.
For instance...
If he wants a toy he can't reach. The older sister can sense his need before he has a chance to be verbal, and give her "big sister' assistance. Unlike her need to get your attention for everything at that age, he has no need to develop his vocal skills. He has a non verbal communication with her and his verbal communication with you will come along more as it is needed.
It is NOT abnormal...it is just not as obvious as other parents may describe. Some older siblings will answer for them, ask for things for them, tell mommy what he feels, wants, needs....it gives the older sibling a sense of importance in the family to be their subtle guardian. But, also delays their communication a little by being their voice.
No worries....
He will develop just fine at his own rate when he needs to...and you'll see, he will talk just a well as anyone else.
If you are overly concerned....start to teach both of them sign language. She can help teach him and make him sign her before helping him...Children can communicate thoughts long before the mouth can. Sign language is used in many day cares and preschools for toddlers.... Words like more, yes, no, want, please, thank you, hungry and thirsty. They can communicate much earlier than people realize. Toddler talk is not the same from family to family...but signing is universal.
Or
Give the big sister an important job....playing the game of 'what's this' and "who's that" all day....teaching him to say words.
Although it looks like a downfall....be happy. It most likely means she likes baby brother and is helping and babying him....not pinching him behind your back and plotting a way to make him disappear.