Not always.
My sister had a baby at 20. She did not marry the boys father. She bought her house at age 23.
She now owns two house, both valued at 350k. The baby is now 26 with a BA from Case.
My nephews family got involved with him at 6 months, they could not resist his face :) Dad was not far behind.
My nephew has the best of both and so do we, good families that love each other out of what could have been ugly.
Never say never my friend.
Your sister is not the norm with unwed mothers. The vast majority end up on public assistance of some type and go into further bad relationships.