Coloring with 16 Month Old

How about making a batch of edible fingerpaint and let him use that to paint while his sister colors?
Here are some suggestion for making it:

Sweetened Condensed Milk Finger Paint
•Sweetened condensed milk
•Food coloring or brightly colored fruit juice
Pour a small amount of sweetened condensed milk into several small bowls. Mix in a few drops of food coloring in each bowl to create a variety of colors. Help your toddler use the colored sweetened condensed milk to pain on newspaper, butcher paper, or craft paper.

Pudding Finger Paint
•Chocolate or vanilla pudding
Make some chocolate or vanilla pudding and allow your toddler to paint with it on a piece of paper. This is a fun activity to use with a print out of a teddy bear outline to let your toddler color in the teddy bear with chocolate pudding. Add food coloring to vanilla pudding to create fun colors.

Corn Syrup Finger Paint
•Light corn syrup
•Food coloring
Pour a small amount of light corn syrup into several small bowls. Mix a few drops of food coloring in each bowl to create your favorite colors. Use the corn syrup finger paint to paint on newspaper, butcher paper, or craft paper. For a fun variation, use this finger paint to paint on the shiny side of a piece of foil.

The crayola begginings line is great! They have flat marker tips that they can't bite off and they don't come with annoying lids, but still won't dry out! I love this line! It's more costly, but totally worth it! I run an in-home toddler care, and these have saved my sanity. :)

How old is your son? Can you find an activity that he can do in the same room, while you and your daughter color? My first suggestion would be to put him in a high chair or booster with some activity so he can be near you and engaged, but not be able to bite the markers and take away too much from your time with your daughter...?

Have you seen the triangular shaped crayons? Or the ones with a ball like holder for him to hang onto better? He's still pretty little....

I agree with another poster. We love the Crayola twistables.

We like Crayola Twistables... they're crayons in a hard plastic pen like thing and the crayons twist up and down... so you could set it at just barely level or all the way in... and he can chew/suck on the plastic as much as he wants.