"David's house ... before him. His throne"

# Your house and kingdom will be confirmed forever before you. Your throne will be established forever.

These two sentences have similar meanings and emphasize that David's dynasty will last forever. 

# Your house and kingdom will be confirmed forever before you

"You will live to see me establish your family and their rule over the people of Israel forever"

# Your throne will be established forever

"I will cause your descendants to rule over Israel forever"