My sister is 15 weeks pregnant mom died from aneurysm she had blood clot in previous pregnancy is clexaine or aspirin best option doctors are unsure?

Judgement call: Clexaine (enoxaparin) is far more potent than aspirin but with potency comes the risk of adverse bleeding. Has your sister been tested for thrombophilic (clotting) disorders? The decision to use/not use this med is a judgement call - no textbook will provide the answer, & depends on your sister's specifics. You need to trust her doctors or switch.