This is my first real post, hello all!

I'm currently GMing a game with 2 Lvl4 Sister Militants wearing equivalents of best-quality Lidhl Power Armor (AP6, Str +10, size: average) and their Hospitaller friend from St. Encarmine (bolt pistol & guard shield + a mix of Mesh & carapace armor, for what it's worth.)

(i) I couldn't find the Bulging Biceps talent anywhere in their trees, and was going to make it available as an elite advance to Sister Retributor for 150xp. The sisters have tried to argue that their power armor makes the talent redundant for them. I tell them the Auto-stabilization trait is more of a Terminator armor thing, but then they point out the Sister in the Soulstorm intro cinematic walking around with a heavy bolter. Part of me thought to give them a heavy weapon with the Compact modification, but the RAW is only for basic & pistol weapons.

(Default heavy weapons for SoB of course are heavy flamers, multi-meltas, and heavy bolters, and so this really only matters for firing the heavy bolter unbraced, with the rest being single fire anyways.)

Thoughts?

(ii) Also, has anyone ever statted the Storm Bolter the community at large has accepted? I've been using 2d10x for damage thus far for their valkyrie/rhino-gunning.

(iii) Does weapon Penetration work on Toughness?

Thanks!

Okay,

first the easiest one, penetration does not effect toughness, only armour.

I would make a storm bolter a twin linked bolter, if you use the AA variant 2d10 X is good, though I would also give a +10% to hit beacause of the Twin Linked.

As for you main question I would say: Yes and No.
As long as the Sisters in Power Armour are standing still on firm ground yes, anything other than that...No

Ow btw, the Witch Hunter 40k codex says that Sister armour does not confer a strength bonus...

You might want to take a look at Luddite's thoughts about Power Armour in the Download section of Dark Reign. Essentially, it all comes down to the quality of their suits. Well-maintained ones from shortly past the DAoT will have pretty much every convenience existing within the rules (including the awesome Auto-Stabilised trait...) while those that are handed over to the "new girls" just past their novitiate might require a little time to engage their stabilisers and be a little wonky when it comes to movement (aka: they use the Brace action like everyone else but don't need a bipod or conveniently placed wall to brace their weapon against).

Ow btw, the Witch Hunter 40k codex says that Sister armour does not confer a strength bonus...

Which, of course, is utter bull - the strength enhancement is one of the main reasons why you create powered armour at all, with even 21st century prototypes increasing human strength five-fold.