Relative or not once you open your home as a business then your sil will be expected to read and sign the same contract as everyone else. It is normal to expect at least part time pay during a mat leave and especially if she is expecting two spaces when it is over.
Also realize by the time she is ready to return it is quite likely that one of the other moms of one year olds you sign on will get pregnant and go on mat leave. We have quite a turn over in this business so it may all work itself out.
As long as you have that extra space you didn't intend to fill you will have a bit more leeway in the sense of being able to accept the extra child. After you have had a chance to get started in daycare you may find you are able and willing to increase the number of children.