Need help gaining weight @ 39, its hereditary in my family..?

I'm 5'4 WEIGH 110 AND 39 YEARS OLD, 3 CHILDREN, 18, 13, 9. I'M REALLY THIN AND KIND OF LOSING MY MUSCLE TONE. I WANT TO STAY FIRM, SO MY 40 SOMETHING HUSBAND DOESNT GO LOOKING FOR THAT YOUNGER BEAUTY. THANKS FOR ANY HELP.