How do you keep the peace between your preschooler and younger toddler?

I have a 16 month old and a 3.5 year old and they have been at each other's throats recently. My younger one is into everything my older one is doing, and becomes laser-focused on being right there with her in the thick of it. And she's getting quicker and more assertive about it by the day. She adores her big sister, which is sweet and all - but her constant attention drives my older one crazy. My older one is really easily overwhelmed and feeling like her little sister is trailing her non-stop is leading to lots of meltdowns. Today I felt like each kid alternated losing it over something the other one did or didn't do.

I can distract my younger one for a bit, but if she sees her big sister doing anything else, she zeroes in on it and jumps right in. Both kids do well with things together outside - running around, bubbles, playing in the yard, things like that...but it's hot and I can't keep them out all day.

Any tips or tricks? Is this just a grin and bear it kind of phase? Three has been a very challenging year for us with my older daughter, and as my younger one is turning into more of a toddler than a baby I'm sensing more trouble brewing.