Depending on how your FIL's will is written and possibly the intestacy laws in FIL's state of residence, you may want to look into a partial disclaimer, also sometimes called a renunciation.
You can also give more than the annual gift limit. First, you both can give her $15K (or whatever the limit is each year), so you can effectively give her $30K per year. If she is married, you can both give her spouse $30K per year too.
If you want to give more than the limit, you can do so, but to avoid gift taxes you'll have to file a Form 706, and it will eat into your lifetime exemption (which may or may not be a problem in the future, depending on what they do with the estate tax exemption and what your net worth grows to). Also, states sometimes have lower estate tax exemptions than the federal level.