I (30f) don’t believe in borrowing money but my fiancée (30m) wants to borrow $150k from his brother for our down payment

I’ve been saving for 5 years prior to meeting my fiancee. A long time ago I asked fiancee how much money he has saved up for a downpayment and he said he could put down $300k.

Turns out that $300k is shared between him and his brother. So technically we’ll be in debt for $150k after marriage if we were to use the whole $300k.

I’m upset that he never told me about this. He makes a great salary and he could’ve easily saved up more then $150k on his own without dipping into his brothers funds but he never made an effort to save in the entirety of our whole relationship because he prefers to live in the moment.

I don’t believe in borrowing money from others because i’ve always believed that if your a responsible human being who takes responsibility for oneself and has common sense then there’s no need to borrow money unless it’s from the bank.

I don’t want to be stuck having kids with him with a mortgage PLUS paying his brother back over my head.

I’m not sure if I’m being overly concerned with this whole not borrowing money thing or if I should just let it go. Any advice would be appreciated.

TLDR: I (30f) don’t believe in borrowing money but my fiancée (30m) wants to borrow $150k from his brother for our down payment