I don't know how to simplify resistors which have 2 grounds
Start by using source transformation like this: -

I've split the input signal into two identical signals then use thevenin's theorem to produce an equivalent source voltage of two-thirds of \$V_i\$ feeding through the parallel combination of the 1 ohm and 2 ohm resistors that joined at \$V_x\$.
You then simplify the circuit so that the resistors surrounding \$V_x\$ become 1 resistor of 1.6667 ohms fed from two-thirds of \$V_i\$: -

Then do another thevenin's equivalent with the 1.6667 ohm resistor and the 2 ohm resistor that connects to ground. That gets rid of your resistors going to ground and you will be left with a simple summing op-amp circuit.
Can you finish this yourself?

I simulated that, but the voltage graph was not sine wave. Just weird.

Taking the step to do a simulation is something I always recommend and well-done to you for that. The modern world is here; step into it. Yes, a real (simulated or really real) op-amp with such low impedances will get into a big mess. If you changed all the resistor values from Ω to kΩ it would certainly give it a better chance of working.