Sinistcha Team Advice for VGC Battles

Sinistcha is best as a support, if you aren't wanting it to be the star of the show then you could pick a solid attacker to pair it with. Use redurection to keep them safe and pumping out damage.

Maybe try pairing it with mega staraptor. Mega starsptor can stack defensive stats with close combat because its ability turns it into a defense boost, so sinistcha healing will be extra effective as the game progresses.

Don't lead with sinistcha, bring it in as your sweeper gets damaged, then keep it in to support with move redirection.

what about Mega Garchomp, Tyranitar, Mega Staraptor, Sneasler and incineroar?

I can't tell you why, but nobody runs mega garchomp, but everybody runs base garchomp. Based off the meta you're better keeping staraptor as your mega, and having a more flexible item on garchomp.

Sneasler and incineroar are kind of overlapping I think. Both acts as fake out support, but it'd be better to choose on of the two and include some special attackers to reduce intimidate vulnerability.

It helps if you have a clear idea for each teammate.

Sinistcha - Redirection, healing, spread damage. Keep the mega alive.

Staraptor/garchomp mega - Physical sweeper, primary win condition.

Tyrannitar - 5/6th slot, pick to mess up enemy weather plans, knock off support.

Incineroar - Intimidate (overlaps with staraptor), fake out (overlaps with sneasler), pivot support.
Sneasler - fake out (overlaps with incineroar), fake out, great burst damage with unburden against intimidate.

Aside from sinistcha there's no other special attackers in an intimidate crammed meta. Consider picking up a milotic to profit from intimidaters.

sinistcha fits pretty much into any team you want to put it in. the first question is what mega are you wanting to go with and why, then build from there