I am looking for mangas with the trope In Love with Family Member's Spouse, for example, a brother that passed away and left behind a lover and the younger brother is in love with him. A good example is Ani ga Nokoshita Koi no Yukue, in which the younger brother tries to take care of the lover. Ikotsu no Tabiji is another good example, in which the brother goes on a trip with the lover. Something like Haru no Noroi (straight couple) is also ok, like the sister passing away and the other sister having to handle her feelings for the boyfriend. Another one is Kimi to Hasu no Niwa de, in case someone likes this type of manga too.

Thank you!

I have a few mangas like that.
First it's "Yosugara Kimi o Omou" https://www.mangaupdates.com/series/ipwqp3v/yosugara-kimi-o-omou, it's similar to "Ani ga nokoshita...", with a omegaverse and taking care of brother's spouse trope.
Then I like "Gikei no senaka" https://www.mangaupdates.com/series/hmmvcmc/gikei-no-senaka, where we have a brother in love with his late sister's husband.

I hope I helped even if a little.

To be honest, sometimes I find searching on Baka-Updates pretty useless. I have been searching for a manga I've read years ago in which there was a man in love with a guy who was his dead sister' husband, but this (in love with family member spouse) category is not showing it.

This is the only one I can think of but I didn’t finish it:

Kyuuhankagai Fukurokouji

Maybe the “dead family member” tag that is has, could help you.