Dont peek chokes, don't peek wide sniper sidelines, you can win most uber fights if both sides have even numbers and percentage because snipers don't do much in uber fights and the medic will have less protection if a scout is sniping.

Probably send in a sac and have the person that sacrificed counter snipe

this is the boring way

Focus the flank. They will generaly be slower to help their flank players because either a scout or roamer will usually be the one on sniper.

bomb that nigga

If they run a sniper at mid you can try being overly aggressive, unless their sniper hits every shot your 6th player will have a greater impact in the fight, and the opposing team will have to fall back, situation in which a sniper isn't as useful.

you just gotta play slower and more carefully. be constantly aware of the sniper and its fine to push the way obviously out of the sniper's sightline (unless their combo is close some other place). pressure flank more and maybe rotate heals there to get a pick. otherwise make sure to not let their sniper get in too far by spamming entrances when you have to hold far back

let me on your team i will go spy and deal with him for you

don't give him/her/they a shot and kill them asap

the moment you get a pick (if its not the sniper), I would recommend soloing in if they are close to you and after you make them force you go behind and pick the sniper and stay as a distraction, that way your team can flood in or you can set up a pinch. this is one of the best ways to play as a team against sniper. the pocket or roamer is best suited for this