Sitting up

What are some thing my fiancé and I can do to strengthen our little ones back to help her sit-up? It’s going to take her a little while longer to be able to do it because she is 5 months old and a little over 20 pounds (she was 9.7 when she was born) so the Dr said it may take her a little longer. She also is still struggling in the holding her head up department. I know it’s bad to say, but, she just has a big ole head and I’m assuming it gets heavy cause she is a big baby.