Sivir has just one role: kill many creeps fast
no build in slow like many other heros
no stune like many other heros
not very fast compared to other heros
low damage compared to other heros
A feeding player is bad equal which hero he plays. So it does not matter, that it was Janna and Anivia (both realy hard to play well heros btw!!).
So let's check your team
Cho'Gath => AE knockup, AE silence, very high HP, decent damage
Blitzcrank => very high PBAE damage, pull, knockup, decent tank
Tristana => very high damage, very good escape abilitys, knockback
Annie => very high damage, tricky stune
Veigar => posible best AE stune, extrem damage
You said, two of you had been feeded ... so they should have more levels and better equip.
Janna => low damage, tricky AE knockup, slow, shield, heal, low health, fast
Anivia => wall, tricky AE stune, if outleveled low damage, low health
Udyr => decent damage (there are many heros with much more), singel target stune, fast
Syvir => medium to low damage, slow, medium health, no(!) disables, nice ulti
Twitch => slow, extrem damage, extrem range, stelth
Janna and Anivia had been bad players as you said. So you faced 3 where one is for sure no real thread against your team (Sivir). Leftover are Twitch and Udyr, both considered as strong to OP.
Udyr is fine as I can say, Twitch isn't OP but NB . NB := Not Balanced.
With your disable power AND damage output, you should have been able, to anihilate the 3 leftovers.
Sounds to me, as you just faced better players then you are .