How would I say this

Thank you in advance for helping me with this.

What would be the proper translation of "Thank you for letting me join your family".
Or something close to it, a phrase that I would say to my fiances family after we are married?.

I take this s a formal sentence you want to say to your in-laws.

-Kazoku no ichi'in to shite mukaete kudasatte arigato gozaimasu.
(Thank you for accepting/welcoming me as a member of the family.)

Thank you so much!.
How would I write that in proper hiragana if I may ask?.

Japanese native people won't say "Thank you for letting me join your family", especially if you are a male.

I would probably say "XXX-ke no ichi-in ni narete ureshii desu"(I am happy to be a member of XXX family", if you are a female(male won't say this unless you are an adopted person), that sounds more natural to me.

"XXX ke no ichi'in ni narete" truly sounds like you became a part of an XXX family institution, and nowadays females would not use that either.
I wrote the sentence to say thank you for ACCEPTING or WELCOMING me, which is a big thing for Japanese parents of certain traditional leaning (as I experienced when I decided to come home with my non-Japanese boyfriend then), and a more formal way to say thank you for allowing me to marry your daughter.

Though i do appreciate the second option, I do feel AK has the better option for my situation. Thank you truly.

Japanese writing, in mixture of hiragana and kanji, by the way, is as follows:

家族の一員として迎えて下さって、ありがとうございます。

-Kazoku no ichi'in to shite mukaete kudasatte arigato gozaimasu.
(Thank you for accepting/welcoming me as a member of the family.)