This is all estamating
So if we think about it, lets say the eldest sister was married for 2 years which means she was 17 when widowed + 3 years mourning= maybe 20-21 at the start of the show .
And Du Yuxi is younger than her which makes him 20-21 when they got married, their age gap would 1 year.
Du Yuxi-20
Elder sister-21
Moving on we have second sister and her husband who I reckon are both 20 at the start of the series.
Second sister - 20
Fan Lianghan- 20
With Chai An I think he his 20-22 years old as he is older than his cousin but younger than Shen Huizhao. Kangning and her twin is around 18 at the start and I think the reason she calls Qiongnu older sister is because they family has always joke she is the sister in law and since Kanning is the younger twin she would have to address Qiongnu as older sister.
Twins age -18
Qiongnu-18
Chai An - 20-22
For the fourth sister it's quite confusing because if we are going by the start of the series I believe she's 15-16 but if we go by the age she married Shen HuiZhao she would be around 17-18 and we know for a fact he is quite a bit older than her compared to the others but since this is a historical drama and people believed 50 was an old age, I would say Shen Huizhao is 22-24 maybe older but he is similar to Chai An so it shouldn't be past 30. So their age gap is like 4-6 years.
Fourth sister - 17-18
Shen Huizhao - 22-24
Lastly, we have the fifth sister and her husband, this one is a bit weird because if you think about it at the start he goes for Kangning which would mean he is her age or older like 18-22 and the fifth sister would be at most 15-16, but if we are talking about at the start of their marriage I reckon she would be 16-17 and he would be 19-21 and than there's the time skip which is three years so when they reunite she is 19-20 and he should be 22-23
fifth sister-15-16
Her husband-18-22
I can't be bothered calculating their ages at the end but they should all be either early 20s or late 20s
