is the second always the hardest?!

My baby girls is 5 months and my husbands want to try around January-March so if I got pregnant they'd be 18-20 mo the apart.....is that a good age gap? Is is hard? Is the second the worst? How do the get along? I was a o my child but iv always wanted a lot of kids but is that too soon? I like the ages of my SIL son and my girl he'll be 3 in Jan and my girl will be 1 in April so there about 2 years apart and I think that's good ages