Forts with blankets, sheets, pillows, card tables, chairs
"Hotlava" kids move around the house without stepping in the hot lava (the floor) - use couch cushions, mats, etc to step on to avoid the floor,
Nerf basketball, nerf "dart"board,
Double sided easel with large paper and giant crayons & markers (I'd avoid paint personally) - if the easel is too expensive buy rolls of butcher paper and roll out on the floor.
blocks - either duplos, wood, plastic. With 2 boys you'll need alot of blocks but it's great for fine motor and spacial skills. Encourage older brother with a timer to see how much time he can devote to younger brother by allowing him to build onto big-brothers blocks creations (since younger brother will want to and big brother won't want him to the timer will give older brother a goal -"yesterday you were so grown up you let Sam build with you for 10 minutes - do you think we can get to 15 today?" TEach him how to use the timer).
(Timers are great to get them to beat their own time at clean-up too)
sock puppets
safety scizzors, glue sticks, construction paper and whatever else is in the discount bin at the craft store (pompoms, yarn, google eyes, pipe cleaners) and sit them at the table (we always had a kid size table with kid size chairs under the dining room window) let them create then write a short description at the bottom of each page.
kiddy pool, sprinkler & water guns with a beach umbrella in the back yard. Grandma can put her feet in the pool & sit under the umbrella while the kids go back & forth between kiddy pool and spinkler (sprinkler will fill the pool gradually)
good luck to you and your mom! Teenager to come over for a few hours is a good idea too! Even a pre-teen who will be cheaper and more willing to play wth the kids.