This is the house that she once lived in?

[keannu]

Is it possible to say this?

This is the house that she once lived in.
I don't think that can replace "which" when it's followed by a preposition like "in", then it's also possible to say

This is the house in that she once lived. => This sounds weird.

I need someone's help.

[Jaskin]

Please note I'm not a teacher nor a native speaker;

This is the house that she once lived in.
This is the house in which she once lived.

This is the house in that she once lived. => This sounds weird.

it does but you could say:
She once lived in that house.

hope it helps
Cheers

[engee30]

You could also say:
This is the house where she once lived.

[keannu]

I didn't get any clear answer, please someone reply to this!

[engee30]

Is it possible to say this?

This is the house that she once lived in.
I don't think that can replace "which" when it's followed by a preposition like "in", then it's also possible to say

This is the house in that she once lived. => This sounds weird.

I need someone's help.

And mine is fine too.

This is the house where she once lived.

[Raymott]

Is it possible to say this?

This is the house that she once lived in.
Yes, that's normal
I don't think that can replace "which" when it's followed by a preposition like "in", then it's also possible to say

This is the house in that she once lived. => This sounds weird.
It is weird. You can't use "in that" in this context. You must use "in which".
I need someone's help.

R.