Affirmative: You are in love with her.
- With whom are you in love?
- Who are you in love with?
Are the both question forms correct?
Yes, they're both perfectly correct in terms of grammar.
The difference is in formality: the first sentence is very formal – you wouldn't hear it in everyday speech or writing as it's very elevated in style – and the second is neutral / slightly informal, and very commonly used.
Question 1 is correct. Question 2 would be more correct if you replaced “who” with “whom”- the question is answered by saying “I am in love with X.” X is an indirect object. “Whom” is the pronoun you use for an object (either direct - as in, “Whom does she love?” or indirect- as in these questions). “Who” is used for a subject (as in, “Who loves him?”).
Question 1 is correct by all measures I am aware of.
Question 2 has two problems in terms of strict grammar, but it is completely normal as informal speech.
The first problem is that the pronoun "who" is the nominative. You need the accusative, as it is the object, so it should be "whom".
The second problem is the old issue of a sentence ending with a preposition. Formal grammar generally says that should not be done.
Fix both, and you end up with the first sentence. However, people who insist on both rules are unusual in my experience. In fact, people who insist on either are pretty rare.
Basically, both are fine, but some people will have a problem with 2 because they have expectations based on grammar as defined by people who wanted English to have a formal grammar structure that it simply doesn't.
"You are in love with whom?" is also acceptable - switching up the sentence order is not required for a question. The interrogative syntax, "with whom are you in love?" is not acceptable for an assertion - "Rachel you are in love with" is not a grammatical sentence, but the 'normal' syntax for an assertion can be used for a question. It subtly changes things, and wouldn't be seen as natural in all situations, but it can be.