I would ask your doctor what modified bed rest means - i.e., can you attend your shower if you sit for the whole time. Not to scare you, but I have a friend who was pregnant with twins and on some type of bed rest (not sure if full or modified) but she attended her shower (she sat the entire time), and then she delivered her babies a week later very prematurely (thankfully, her babies are both fine). So, I would absolutely defer to your doctor's advice.
Worst case scenario is the shower can be held without you (I know, that's no fun!) and the gifts delivered to you (or maybe your SO can go in your place) or it can be changed to a meet the baby party after the babies are home and healthy enough for visitors, and your friends have settled in enough with their LOs to feel ready to host something.