"Modulate" is a color multiplier, so there is no way to make a sprite brighter with it. It is a bit hackable though, because you can input colors higher than 1 by toggling RAW mode on (usually for HDR and bloom). This will brighten your sprite, but it won't work if your sprite is too dark, or black for example.
If you are looking for a cleaner solution, you can use a shader to interpolate the sprite regular texture and a white color, with a uniform
parameter:
shader_type canvas_item;
uniform float whitening;
void fragment() {
vec4 texture_color = texture(TEXTURE, UV);
COLOR = vec4(mix(texture_color.rgb, vec3(1,1,1), whitening), texture_color.a);
}