Well if the other rat is male...it will be quite obvious!
The testicles are large and male rats have no shame!

When you say your sister's rat was "supposed to be a boy but had girly parts" - what do you mean? Like nipples?
Pregnant rats will take on a pear shape.
The nipples will be noticable.She may also nest.
How long did you have your sister's rat with yours???
Rat gestation is generally around 21-24 days.
She could just be fat.
What do you feed her?
Also you should get a female friend for your rat.
They do much better in pairs!