There is NO gift tax — UNTIL the ultimate combined estate and gifts previously given total $11.2 million! (That’s the current law, anyway. It could be lower in the future.) So I’m guessing that this won’t apply in your situation — that your MIL’s estate won’t be over that $11.2 million level!
So don’t worry about gift taxes. Be very grateful, and follow the instructions of your mortgage broker. (They need the gift tax return to prove that this isn’t a “loan” adding to your indebtedness.) Other than that there is no issue with this gift.
And by the way, she is entitled to gift each of you $15,0o0 per year, exclusive of the combined estate and gift tax. So remind the broker that the truth gift in excess of the combined $30,000 is only $40,000. Still, for the mortgage process they may need proof that it is all a gift and demand the form include the full $70,000. Don’t worry about that. It won’t ever be an issue. How do I know? If she had over $11M in her estate, she’d probably gift the entire house to you!!
Further thought — please do make sure that she can “afford” this gift. One day, you may feel obligated to have her come and live in your guest room.