"И если они упадут с крыши?"
Перевод:Et s'ils tombent du toit?
I cannot speak very well Russian yet and I guess French would be hard for you to understand in that precise context:
We use "s'ils" or "s'il" because si-il would make a hiatus in the sentence. In "si elles", the e that follows the i sounds more natural.
It is the same in
"Je t'aime" Which would normally be "Je te aime" Or "l'amour" which would normally be "le amour" It is called elision, we use it mostly for pronouns, articles, negation