'glad i' expresses non-romantic affection/fondness for someone/something, whereas 'Jeg er glad' would mean 'I am happy'. And in terms of English, "I am happy of my siblings", is incorrect. You could say you were happy for your siblings, or happy with your siblings, but not happy OF your siblings.
My Norwegian wife explains that in practical usage "elsker" is really reserved for your spouse or an individual one is romantically involved with. For others (like siblings, friends), the phrase to express affection is "glad i" but it is not an insult to use this phrase with your spouse as well. The usage is akin to "I appreciate you" combined with "I enjoy (being around) you"