Should I be worried about my 2 and a half year old sister?

Ok, I'm 19 and have a 2 and a half yr old half sister. On my last visit to see her I took a friend with me (who happens to work with toddlers), who was shocked to hear my sister is still bottle fed formula, uses her pacifier and can't talk (the only words she says are mama and dada) I must admit, as much as I love my stepmom she is very lazy & would rather sit online than interact with my sister. Is all this normal?! I'm not sure whether to say something, I don't want to cause arguments.