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If by giving consent, her sister is saved from sin and poverty and staying single and not becoming a mother, then what a Mustahab act can you find better than sharing your husband with her?
have a look a these narrations:
 بإسناده عن عليّ عليه السلام قال قال رسول اللّه صلّى اللّه عليه و آله كتب اللّه الجهاد على رجال أُمّتى، والغيرة
على نساء أُمّتى، فمن صبرت منهنّ واحتسبت أعطاها اللّه أجر شهيد

"On the authority of Ali (peace be upon him), he said that the Messenger of Allah (peace be upon him and his family) said: Allah has prescribed jihad for the men of my nation, and Ghayrat(jealousy about having a co-wife) for the women of my nation. So whoever among them is patient and endures with the intention of seeking reward, Allah will grant her the reward of a martyr.
العدة ، عن البرقي ، عن محمد بن الحسن ، عن يوسف بن حماد ، عمن ذكره ، عن جابر قال: قال أبو جعفر عليه السّلام : "غيرة النساء الحسد و الحسد هو أصل الكفر إن النساء إذا غرن غضبن و إذا غضبن كفرن إلا المسلمات منهن".
The jealousy of women is envy, and envy is the root of disbelief. When women become jealous, they become angry, and when they become angry, they disbelieve, except for the believing women among them."

Of course everyone should know that it's not obligatory for you to give consent and it's better that men understand their wives' emotions and as long as the first wife has not fully given her consent they don't go for the second. because they should not ruin the first life for helping another sister.
All said above are recommendations and not obligations
Wassalam