My DS and DD are 21 months apart too. My stepmom stayed with us the week before DD arrived and for two weeks after. She was a life send in helping me care for DS. I was able to rest and focus on bf DD while she played with DS, fed him, baths, etc.
My DH only was able to take off work while we were in the hospital, so I really needed help. If your DH will be home, you may not need as much help as I did.
Also, if the help would have been my mil I probably would have gone insane. She is no help at all. When DS was born, she made things worse. Cried when he cried, stood around and stared at me, ate our food, etc. So, I'd understand if you said no help would be better than your mil...